Characterisation of the effect of intravenous infusion of glucose and tolbutamide on the insulin delivery rate in man

Diabetologia, Sep 1978

Summary Serum insulin response to a single bolus of IV glucose or tolbutamide was measured in eight healthy subjects. Insulin disappearance rate was assessed by deconvolution from the serum insulin levels, using the measured insulin disappearance rate. The mean rate constant of insulin disappearance was 0.238±0.005 min−1 (mean±SEM). Basal insulin delivery rate was 8.0 to 9.0 mU/min and the delivery rate following glucose injection (0.5 g/kg body weight) showed a biphasic response, whereas that after tolbutamide injection (15.6 mg/kg body weight), a monophasic response. After glucose injection, 1.7±0.3 U of insulin was delivered during the first phase (0–10 min) and 5.6±1.6 U during the second phase (11–60 min). After tolbutamide injection, 1.5±0.3 U of insulin was delivered during the first 10 min. Between 11 and 40 min, 1.6±0.5 U of insulin was delivered. The results thus confirm and also quantitate biphasic insulin secretion after a bolus of glucose with a monophasic response after tolbutamide. The method is suitable for studies of the insulin secretogogues in man.

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Characterisation of the effect of intravenous infusion of glucose and tolbutamide on the insulin delivery rate in man

Characterisation of the Effect of Intravenous Infusion of Glucose and Tolbutamide on the Insulin Delivery Rate in Man T. Asano 0 H. Sasaki 0 M. O k u m u r a 0 0 First Department of Internal Medicine, Fukuoka University,School of Medicine , Nanakuma, Fukuoka , Japan Summary. Serum insulin response to a single bolus of IV glucose or tolbutamide was m e a s u r e d in eight healthy subjects. Insulin disappearance rate was assessed by deconvolution f r o m the serum insulin levels, using the measured insulin disappearance rate. T h e m e a n rate constant of insulin disappearance was 0.238 _+ 0.005 min -1 (mean _ SEM). Basal insulin delivery rate was 8.0 to 9.0 m U / m i n and the delivery rate following glucose injection ( 0 . 5 g / k g body weight) showed a biphasic response, whereas that after tolbutamide injection (15.6 mg/kg b o d y weight), a monophasic response. A f t e r glucose injection, 1.7 + 0.3 U of insulin was delivered during the first phase ( 0 - 1 0 rain) and 5.6 + 1.6 U during the second phase ( 1 1 - 6 0 min). After tolbutamide injection, 1.5 + 0.3 U of insulin was delivered during the first 10 rain. B e t w e e n 11 and 40 min, 1.6 + 0.5 U of insulin was delivered. T h e results thus confirm and also quantitate biphasic insulin secretion after a bolus of glucose with a monophasic response after tolbutamide. T h e m e t h o d is suitable for studies of the insulin secretogogues in man. Insulin delivery rate; insulin secretion in man; glucose; tolbutamide; insulin disappearance rate; biphasic insulin secretion - Studies with isolated perfused rat pancreas have demonstrated that glucose produces a biphasic release of insulin, whereas tolbutamide produces only a monophasic release [ 1, 2 ]. T h e most commonly used technique for studying the release of insulin from the h u m a n pancreas has b e e n simply to measure the peripheral plasma insulin levels [ 3-5 ]. This technique is not sufficiently sensitive to observe any difference in the response pattern between glucose and tolbutamide, unless direct assay of insulin level in the portal vein is used [6]. M o r e recently, T u r n e r et al. [ 7 ] have followed the rate of insulin removal from the h u m a n plasma after insulin injection. Combining these data with the m e a s u r e m e n t of plasma insulin after administration of glucose allows a m o r e accurate estimate of the insulin release rate. Using this technique and the assumption that the rate of removal is independent of the insulin concentration, these authors have shown a biphasic release of insulin [ 7 ]. This m e t h o d has not yet been applied to stimulating agents other than glucose and xylitol [ 8 ]. In the present study, the effect of glucose and tolbutamide infusions on the insulin delivery rate was c o m p a r e d mostly in the same subjects. Materials and Methods Eight healthy volunteers, aged 21 to 58, were studied after an overnight fast for the determination of 1) the insulin disappearance rate and 2) the insulin delivery rate followingglucose or tolbutarnide infusion. All of the subjects were within 89-110% (average 105%) of their ideal body weightfor age, height and sex, and non-diabetic and had no family history of diabetes mellitus. They were not taking any drugs at the time of the study. Subjects were encouraged to follow a weight-maintainingdiet with at least 40% of the caloriesas carbohydrate for three daysbefore starting the experiment. An antecubital intravenous catheter (Venula catheter ~2, gauge 21), connected to a three-way stopcock, was inserted in a large vein for blood sampling and was kept patent with saline solution. Insulin disappearance rates in eight subjects (Table t) were measured after IV infusion of an approximately 0.06 U/kg body weight of monocomponentinsulin (Pork, Actrapid, NOVO Co.,) into the other forearm. Insulinwas infusedwithin5 seconds. To avoid any potential effect of hypoglycaemiaon insulindisappearance, a glucosesolution (20 g/dl) was continuouslyinfused through the indwelling venous needle at a constant rate of 2.2 (range: 2.0-2.7) mg/kg/min (0.76 ml/min by Harvard pump). The glucose infusion was started 60 rain before the administration of insulin and continued throughout the entire period of the experiment. The glucose infusion lasted for 120 minutes. Such a glucose infusion was found to maintain plasma glucose and insulin concentrations slightly higher than fasting (glucose and insulin levels: 91.2 + 2.0 mg/dl and 17 + 3.0 ~xU/ml (mean + SEM) at 55 min infusion, and 94 + 2.0 mg/dl, 16 --- 1.6 ~U/ml at 60 min infusion, respectively). These pairs of values do not differ significantly, indicating that a steady-state had been reached). Satisfactory steadystate conditions for plasma glucose and insulin were achieved within 60 minutes. Blood samples were obtained from the indwelling catheter at 3, 5, 10, 15, 20, 30, 40, 50 and 60 rain after the insulin infusion. The dead space (approx. 0.2 ml) was discarded before each sample was taken. Intravenous infusion of glucose in eight subjects or of tolbutamide in six of the eight subjects were performed on separate days. After collection of two control samples at 5 rain intervals, glucose or tolbutamide solution was infused over 2 rain into the forearm vein with a glucose dose of 0.5 g/kg body weight as 50 g/dl solution (average 32.1g, ranging from 27.5-39.0g) or tolbutamide with an average dose of 15.6mg/kg, ranging from 13-18 mg/kg (total 1.0 g). Blood samples were taken at 3, 5, 10, 15, 20, 30, 40, 50 and 60 rain after the start of the infusion from the indwelling catheter in the other forearm-vein. Clotting in the catheter was prevented by a slow infusion of saline (0.154 tool/l). Blood samples (1.0 ml) were collected in sodium fluoride tubes for subsequent plasma glucose determination. Plasma glucose was measured, on the day of the test, by an o-Toluidine method [ 9 ]. For insulin assay, samples (4.0 ml) were collected into plain tubes and later centrifuged. Serum insulin was determined in duplicate by a radioimmunoassay with charcoal separation of bound and free insulin [ 10 ]. Controls were carried out using tubes with standard insulin and charcoal-treated "insulin-free" serum (fasting human serum) [11]. When measuring samples containing high insulin concentrations, the serum was prediluted with "insulin-free" serum. The precision of the assay over the range of 10-100 ~xU/ml was in the order of 5 % coefficient of variation within assay and 10% between assays. The sensitivity of the assay was 3.8 ~tU/ml using the same criteria as Albano et al [11]. Human insulin MRC 66/304 (24 U/mg, Medical Research Council, London) was used as standard. Calculation o f the Insulin Defivery Rate The estimation of the insulin delivery rate was made by analysing the peripheral serum insulin levels in response to the infusion of glucose or tolbutamide, on the basis of the insulin disappearance rate. The theoretical considerations were the same as those of Turner et al. [ 7 ]. The measurement of the disappearance rate of unlabelled insulin may be complicated by the fact that plasma glucose concentration may be decreased. The influence of hypoglycaemia on the disappearance rate of insulin is generally considered insignificant [ 7, 12, 13 ], even if endogenous insulin is unchanged or suppressed [ 7 ]. Although several authors [ 12, 14 ] have shown that the irreversible loss rate of insulin is not linearly related to plasma concentrations, the difference is not very large and it is reasonable to assume that the fractional rate of loss of unlabelled insulin is relatively constant regardless of the plasma concentrations. Therefore, there are reasonable grounds for assuming that the calculated rate of insulin disappearance following a single dose of insulin can be applied to the delivery of various amounts of insulin following glucose or tolbutamide infusion. The insulin disappearance rate was obtained from least squares analysis of a semilogarithmic plot of total serum insulin levels following insulin infusion. The apparent distribution space (DS) of insulin was obtained by dividing the amount of insulin infused by the concentration difference between the extrapolated level of serum insulin at time zero and the "steady-state" insulin level before exogenous insulin injection [15]. This provided a scaling factor for converting changes in the hepatic delivery rate into amounts of insulin delivered. Half-disappearance time (T 1/2)was calculated using the formula [ 15 ]:T 1/2 = 0.693/fractional disappearance rate, and metabolic clearance rate (MCR) using the formula [ 12 ]:MCR = DS fractional disappearance rate. If Y 1 and Y 2 represent the insulin concentrations measured at time T 1 and T2, respectively, and Y 3 represents the insulin level calculated from the insulin disappearance rate at T 1, then Y 2-Y 3 represents the difference at T 2 between the insulin levels observed and calculated from the insulin disappearance rate. The area surrounded by Y 1, Y2 and Y3 represents the insulin delivered during T 2 - T 1 . Thus, the post-hepatic insulin delivery rate is calculated from the formula: S = f T2 T1 (pT + A)dt - Ae-B(T'T0dt f T2 T1 Where S = insulin secretory rate during T 2 - T 1 in ~U/ml/min, T = time in minutes, P = the slope from Y1 to Y2, A = insulin concentration at zero time, B - a rate constant determining the disappearance rate. At a steady-state of plasma insulin concentration, insulin disappearance must equal insulin delivery [ 16 ]. The fasting serum insulin level before a stimulus represents a steady-state condition and the delivery rate also tends to be constant throughout the fasting period. The calculated disappearance rate of insulin was applied to the fasting delivery rate. The basal insulin delivery rate (F) was calculated from F = A J'~) e-BTdt. The scaling factor was applied to "S" to obtain the insulin delivery rate in U/min and also to " F " for the basal rate. Net insulin values (where the fasting serum insulin level was subtracted from the subsequent insulin levels) were used in all calculations of the delivery rates following glucose and tolbutamide. Results 1) Insulin Disappearance Rate T h e m e a n i n s u l i n c o n c e n t r a t i o n s in e i g h t s u b j e c t s ( T a b l e 1) f o l l o w i n g i n s u l i n i n f u s i o n a r e s h o w n in F i g u r e 1. T h e c a l c u l a t e d first c o m p o n e n t o f f r a c t i o n a l i n s u l i n d i s a p p e a r a n c e w a s e m p l o y e d f o r t h e e s t i m a t i o n o f t h e r a t e c o n s t a n t o f d i s a p p e a r a n c e f o r i n d i v i d u a l s . T h e m e a n 0 . 0 0 5 m i n -1 ( m e a n r a t e c o n s t a n t w a s 0 . 2 3 8 + + S E M ) . T h e i n s u l i n i n f u s i o n r e s u l t e d in a f a l l in p l a s m a g l u c o s e t o 7 3 % ( 6 9 m g / d l ) o f t h e s t a r t i n g l e v e l a f t e r 3 0 m i n ( p < 0 . 0 5 ) . T h e m e a n e x t r a p o l a t e d i n s u l i n l e v e l a t z e r o t i m e p r o d u c e d b y a s i n g l e I V d o s e o f 4 . 0 + 0 . 2 U o f i n s u l i n ( 0 . 0 6 U / k g ) w a s 8 8 5 _+ 7 2 v U / m l . M e a n v a l u e s o f D S , M C R a n d T 1/2 a r e s h o w n in T a b l e 1. 2) Insulin Secretory Rate T h e g l u c o s e l e v e l s d u r i n g t h e g l u c o s e i n f u s i o n ( F i g . 2) r e a c h e d a p e a k o f 3 0 7  12 m g / d l 5 m i n a f t e r Tolbutamide Age 34.5 +_ 5.8 ?U/ml starting the infusion and then decreased steadily with time, whereas, with tolbutamide, glucose concentrations decreased to 51 _+ 2 mg/dl at 30 min after infusion and r e t u r n e d to 69 m g / d l at 60 minutes. M e a n s e r u m insulin concentration after glucose and t o l b u t a m i d e infusion are shown in Figure 3. T h e insulin level following glucose infusion p e a k e d at 122 + 16 g U / m l 5 min after starting the injection (basal level of 7.2 _+ 1.5 ~xU/ml). With tolbutamide infusion, the p e a k m e a n insulin level was 79.3 + 13.0 ~tU/ml 5 min after starting the injection [basal level of 9.0 + 2.0 ~tU/ml]. T h e shaded area b e t w e e n the time of successive samples in Figure 3 was then ! ! ,| i i -5 0 i0 i 40 t 50 60 20 30 Time ( mln ) Fig. 2. Mean blood glucose levels followingan injection of glucose (n=8) (0.5 g/kg) or tolbutamide (n=6) (15 mg/kg) in healthy subjects t a k e n as the quantity of newly secreted insulin in the corresponding intervals on the basis of the rate constant of insulin disappearance. T h e insulin secretory rates calculated f r o m the individual insulin levels following glucose or tolb u t a m i d e infusion are shown in T a b l e 2. O n glucose infusion, the insulin secretory rate showed a biphasic response. T h e first rapid p h a s e a p p e a r e d to last for the first 5 to 10 rain and there was a small decrease in the period b e t w e e n 11 and 20 rain. T h e r e was a slight but significant increase as a second phase b e t w e e n 21 to 30 min (p < 0.02 in c o m p a r i s o n with the rate during 16 to 20 min), and followed by a gradual decline T o l b u t a m i d e (~tU/ml/min) m e a n + S E M a p < 0 . 0 2 } b p < 0.01 v . s . tolbutamide injection p < 0.005 T h e difference of the rate between 1 6 - 2 0 min and 2 1 - 3 0 min in glucose injection was significant (p < 0.02) ~U/ml 120 I 40 I 50 Time ( min ) toward the end of the experiment. On glucose infusion, the insulin secretory rate in all periods was significantly higher than the basal rate (p < 0.01). The insulin secretory rate in response to top butamide showed a monophasic pattern. The rate in the period 0 to 15 min was significantly different from the basal rate (p < 0.01). The values during the 16 to 40 min period were also significantly higher than the basal rate (p < 0.05), but there was no secondary increase in the rate during this period. Forty minutes after infusion of tolbutamide, the rate was not significantly higher than the basal rate (p < 0.1). After the infusion of glucose, the insulin delivery rates between 21 and 60 min were significantly T. A s a n o et al.: Insulin Responses to Glucose and Tolbutamide higher than those following tolbutamide infusion (Table 2). There was no significant difference between insulin delivery rates in the interval 4 - 2 0 min after glucose and tolbutamide infusion. 3) Amount o f Delivered Insulin The amounts of insulin delivered during the various periods are shown in Table 3. If the first phase of insulin secretion after glucose infusion was assumed to start at 5 rain and last for 15 min, then the amount of insulin delivered during this first phase varied from 1.0 to 2.2 U. With tolbutamide infusion, the value during the first phase was not significantly increased over basal, even if the period was extended to include all of the first 15 min. It was considered that the first 10 min after infusion was the most representative period of the first phase in the response to both glucose and tolbutamide. Discussion Studies of the disappearance rate using a single injection of insulin are complicated by the presence of more than one exponential component in the plasma disappearance curve [ 17 ]. In the present study, rate constants from the initial phase were used to calculate the post-hepatic insulin delivery rate. Our estimation of disappearance rate of insulin from the initial phase (T 1/:: 2.9 min) is comparable with that measured directly following the cessation of prolonged infusions of insulin, which is thought mainly to be due to insulin degradation (T 1/2:3 to 4.3 min) [ 12 ] and is also similar to the estimation by Turner et al. [ 7 ]. The DS of 76 ml/kg compares well with those (82 ml/kg) measured following the prolonged infusion technique of MC insulin by S6nksen et al. [ 12 ]. Our estimated space (4.9 1) was smaller than the space (12.6 1) obtained by Turner et al. [ 7 ], a difference only partially accounted for by the fact that our subjects were smaller (64 kg) than those (77 kg) of Turner et al. The accuracy of the estimation may depend upon the dose of insulin administered, different doses seem to give wide variability. Previous studies on the metabolism of unlabelled insulin using continuous infusion techniques [ 12, 14 ] found greater clearance of unlabeUed insulin than labelled insulin [ 12, 16 ]. Our estimation of M C R of 1166 ml/min or 18 ml/kg/min was similar to those measured by others (11.4-34 ml/kg/min by S6nksen et al. [ 12 ], 861 ml/min by Genuth [ 14 ]). In the present study, a similar pattern of insulin secretory response to that obtained from the perfused rat pancreas [ 1, 2 ] was observed in man both following glucose and tolbutamide administration. Although it has been reported that a single bolus of glucose stimulated only an acute insulin release without a second phase [ 4, 18 ], we observed, in this study, a biphasic insulin response to the single short infusion of glucose. The second phase appeared as a small rise during a relatively slow decline of glucose concentration while the glucose level during the second phase (20 to 30 min) was in the range of between 165 and 205 mg/dl. The pattern of insulin delivery rate in our tolbutamide study was compatible with that demonstrated in the perfused rat pancreas without glucose [ 2, 19 ]. The tolbutamide-induced monophasic pattern of insulin delivery could be due to the hypoglycaemia caused by the administration of tolbutamide itself (Fig. 2), since tolbutamide-induced insulin secretion is affected by simultanous glucose administration [ 19, 20 ]. Basal insulin delivery was estimated to be 11.5 to 13.0 U/day, when the mean basal insulin level of 7.2 to 9.0 ~tU/ml was assumed to be maintained over the entire 24 hours. This estimate compares well with previous estimated 12 U/day made by Genuth [ 14 ], 14 U/day by Turner et al. [ 7 ] and 12.5 U / d a y by Sherwin et al. [ 17 ]. The duration of the first phase of insulin response following glucose injection varies from study to study, ranging from 5 to 10 min [ 3, 7, 18, 21, 22, 23 ]. It was difficult to define the duration of the firstphase response with our sampling times. If the early response is assumed to be the same for glucose and tolbutamide, the initial 10 min seems to be most representative of the first phase. The timing of the samples means that although a difference between the second phase to glucose and tolbutamide was apparent, the exact timing of the second phase could not be assessed from these results. Turner et al. [ 7 ] have calculated insulin delivery of 0.23 to 1.53 U in the first 6 min and 1.2 to 3.3 U in the subsequent 54 min. In our experiments, the insulin delivered during the first phase following glucose infusion was similar to that estimated by Turner et al. (using the first 6 rain), but the amount delivered during second phase was greater than their estimates. Some merits of the present method for estimation of the response may be considered in comparison with other methods. The estimation of the response above the basal level or of the area under the insulin response curve is generally used as the index of total insulin response. These methods make the results more complicated [ 24 ]. In the present method, the results give a physiologically meaningful term, stating the rate of insulin delivered to the general circulation, in response to glucose or tolbutamide. The m e t h o d described by Turner et al. [ 7 ] obviously r e q u i r e d t o o f r e q u e n t s a m p l e c o l l e c t i o n s f o r r o u t i n e c l i n i c a l w o r k . T h e u s e o f c o m m o n p r o t o c o l s f o r I V Reterences Dr. T. Asano First Department of Internal Medicine Fukuoka University Medical School Nishiku, Nanakuma Fukuoka City 814 Japan 1. Grodsky , G.M. , Bennett , L.L. , Smith , D. F. , Schmid , F. G. : Effect of pulse administration of glucose or glucagon on insulin secretion in vitro . Metabolism 16 , 222 - 223 ( 1967 ) 2. Curry , D.L. , Bennett , L.L. , Grodsky , G.M.: Dynamics of insulin secretion by the peffused rat pancreas . Endocrinology 83 , 572 - 584 ( 1968 ) 3. Lerner , R.L. , Porte , D. , Jr.: Relationships between intravenous glucose loads, insulin responses, and glucose disappearance rate . J. Clin. Endocrinol. Metab . 33 , 409 - 417 ( 1971 ) 4. 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T. Asano, H. Sasaki, M. Okumura. Characterisation of the effect of intravenous infusion of glucose and tolbutamide on the insulin delivery rate in man, Diabetologia, 1978, 159-164, DOI: 10.1007/BF00421232