Does aging intensify the insulin resistance of human obesity?

Relative insulin sensitivity in aging and obesity was studied during 6-h euglycemic-hyperinsulinemic clamp procedures in seven nonobese weight-matched premenopausal (Pre-M) and postmenopausal (Post-M) women and in nine weight-matched obese Pre-M and Post-M subjects. Peripheral glucose disappearance rates (Rd) and endogenous glucose production rates (EGPR) were measured during baseline (0-2 h) and at insulin delivery rates of 10 (2-4 h) and 40 mU/m2.min (4-6 h). Baseline Rd and EGPR were comparable in the four groups. In Pre-M nonobese women, Rd rose 1.6- and 4-fold above baseline, respectively, at 2-4 and 4-6 h. In all other groups Rd did not change at the lower insulin delivery rate and rose only 2-fold above baseline at the higher delivery rate. EGPR was suppressed 61% and 100% at the two insulin infusion rates, respectively, in Pre-M nonobese subjects. Similar normal suppression was observed in Post-M nonobese individuals. However, in both obese groups, EGPR suppression was comparable and significantly less than that in nonobese groups. These results confirm the presence of peripheral tissue insulin resistance in both obese and aging women. However, in aging, unlike obesity, EGPR is normally suppressed by insulin. In addition, aging does not appear to intensify the insulin resistance of obese states when both coexist in the same individual.

Impact of maternal fuels and nutritional state on fetal growth.

Several maternal plasma fuel abnormalities have been described in gestational diabetes mellitus (GDM), and all may contribute to the development of fetal macrosomia, generally because of the surfeit of calories they provide. Elevated maternal plasma glucose and amino acid concentrations represent key disturbances, because they are also well-known fetal pancreatic beta-cell secretagogues. Fetal hyperinsulinemia contributes to macrosomia in a special way by selectively accelerating fuel utilization and storage in insulin-sensitive fetal tissues. Maternal obesity intensifies the insulin resistance already present in late pregnancy and probably exaggerates the metabolic abnormalities attending GDM that impact on fetal growth and development. However, the means by which maternal obesity per se promotes the development of heavy babies in nondiabetic pregnancies remains poorly defined. Significant correlations exist between newborn birth weight and the levels of maternal plasma glucose, amino acids, free fatty acids, and triglycerides in diabetic pregnancies. However, the relative influence of each disturbance on fetal birth weight remains controversial and requires more detailed investigation.

Insulin resistance in normal rats infused with glucose for 72 h.

Insulin resistance is accentuated during periods of poor metabolic control in human non-insulin-dependent diabetes mellitus. The role of hyperglycemia in this suppression of insulin action is not clear. If glucose impairs insulin action, then the effect should be reproducible in vivo in tissues of normal intact rats. To test this possibility, normal rats were continuously administered 50% glucose in water (60-66 mg.kg-1.min-1) via an indwelling jugular catheter. After 72 h, these animals were hyperglycemic, hyperinsulinemic, and glucosuric compared with control rats infused for 72 h with normal saline (P less than 0.01). Basal glucose uptake in vivo was greater in muscle of glucose-infused rats. Insulin-stimulated glucose uptake in vivo and in vitro (by perfused hindquarters and isolated adipocytes) were suppressed in the glucose-infused group (P less than 0.01). Glycogen synthase activity was reduced 40% in extracts of muscle and adipose tissue of hyperglycemic rats. Basal and isoproterenol-stimulated lipolysis were increased, whereas insulin suppression of lipolysis was blunted in adipocytes from glucose-infused animals (P less than 0.01). Glucose infusion did not alter insulin binding by isolated adipocytes or solubilized skeletal muscle insulin receptors. These results suggest that a 72-h in vivo glucose infusion impaired insulin action in muscle and adipose tissue of normal rats by inducing postbinding defects similar to those observed in human diabetes mellitus during intervals of deteriorated metabolic control.

Effect of increased central blood volume with water immersion on plasma catecholamines during exercise.

To examine the influence of an increase in central blood volume with head-out water immersion (WI) on the sympathoadrenal response to graded dynamic exercise, nine healthy men underwent upright leg cycle exercise on land and with WI. Plasma norepinephrine and epinephrine concentrations were used as indexes of overall sympathoadrenal activity. Oxygen consumption (VO2), heart rate, systolic blood pressure, and plasma concentrations of norepinephrine, epinephrine, and lactate were determined at work loads corresponding to approximately 40, 60, 80, and 100% peak VO2. Peak VO2 did not differ on land and with WI. Plasma norepinephrine concentration was reduced (P less than 0.05) at 80 and 100% peak VO2 with WI and on land, respectively. Plasma epinephrine and lactate concentrations were similar on land and with WI at the three submaximal work stages, but both were reduced (P less than 0.05) at peak exertion with WI. Heart rate was lower (P less than 0.05) at the three highest work intensities with WI. These results suggest that the central shift in blood volume with WI reduces the sympathoadrenal response to high-intensity dynamic exercise.

Levonorgestrel and norethindrone alter insulin action on skeletal muscle of the female rat.

Prospective studies of women receiving oral contraceptives suggest that the progestin component may induce insulin resistance and variable deterioration of glucose tolerance. Because the tissue sites and nature of this insulin antagonism are not well-defined, we studied the effects of two parenterally administered progestins, levonorgestrel (NG) and norethindrone (NE), on insulin-regulated glucose uptake and phenylalanine release by the perfused rat hindquarter. Female rats were injected sc for 14 days with NG or NE (10 or 30 micrograms/kg/day). Low-dose NG and high-dose NE approximate the per kg dose received by women taking a high-dose progestin oral contraceptive. Phenylalanine release and glucose uptake (nmole/min/g) by the perfused hindquarters were calculated from the A-V difference for each. Progestin treatment (30 micrograms/kg/d) significantly reduced phenylalanine release from hindquarters perfused without exogenous insulin. Hindquarters from the high dose NG and low and high dose NE rats perfused with insulin (100 microU/ml) released 22% less phenylalanine than control rats perfused with the same insulin concentration (P less than 0.01) but the net suppression below baseline was similar in the control and steroid-treated groups. High-dose progestin treatment did not alter glucose uptake by hindquarters perfused without exogenous insulin. Insulin (100 microU/ml) increased glucose uptake by hindquarters of control and progestin-treated rats as compared to animals in the same treatment group perfused without exogenous insulin (P less than 0.01). High dose NE impaired insulin-stimulated glucose uptake 24% below values of the control group (P less than 0.01). The other NE and NG doses had no effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Impaired plasma catecholamine response to submaximal treadmill exercise in obese women.

After an overnight fast, blood samples were obtained from seven obese women (50% +/- 3% body fat) and from seven control women (25% +/- 1% body fat) before, during, and after 10 minutes of treadmill exercise at 70% of each individual's maximal oxygen uptake (VO2max). During exercise, peak plasma epinephrine (E), norepinephrine (NE), and glucagon concentrations in the control group significantly exceeded corresponding peak values in the obese group by 1.4-fold to twofold, whereas lactate responses did not differ. After 5 minutes of rest, peak plasma glucose, free fatty acid (FFA), and growth hormone (GH) concentrations in the control group also were significantly higher than in the obese women, but the plasma cortisol responses were comparable. Although plasma insulin concentrations decreased during exercise and rose to maximum values at 5-minute recovery in all individuals, levels were more than 3.5-fold higher in the obese group throughout the study. We conclude that the combination of heightened plasma insulin and diminished catecholamine and other counterregulatory hormone responses may account for subnormal plasma substrate increments that distinguish obese from non-obese women during exercise at comparable work intensities.

Obesity and heredity in the etiology of non-insulin-dependent diabetes mellitus in 32,662 adult white women.

Obesity and family history of non-insulin-dependent diabetes mellitus were examined in a cross-sectional study of 32,662 white women in the United States and Canada who were members of the TOPS (Take Off Pounds Sensibly) Club, Inc., with respect to the association between these two factors and the prevalence of diabetes by using questionnaire data originally collected in 1969. A family history index based upon the reported presence of diabetes among siblings, parents, and grandparents was used to grade hereditary factors on a scale of zero to five. The ratio of actual to ideal weight was used to categorize levels of obesity. The prevalence of diabetes increased consistently with increasing levels of either obesity or family history. The two factors appear to present independent risks for diabetes. Among women with no family history of diabetes, the odds ratio for diabetes associated with a weight 75 per cent or more above ideal in comparison with a weight less than 10 per cent above ideal was 7.2. Among women who were less than 10 per cent above ideal weight for their height, a comparison between women with a family history index of 1.5 or greater and women with no family history of the disease showed an odds ratio for diabetes of 6.0. Women with both risk factors (family history index of 1.5 or greater and relative weight of 1.75 or greater) had an overall odds ratio for diabetes of 22.8 when compared with women who had no family history of diabetes and a weight less than 10 per cent above ideal. Further analysis showed the combination of these factors to have a positive synergistic effect. The interaction follows a multiplicative pattern.

Effects of parenteral testosterone administration on insulin action in perfused hindlimb muscle of female rats.

The effects of 14 days of testosterone (T) administration (1 mg/kg/d) on insulin-regulated phenylalanine release and glucose uptake by perfused hindquarters of intact and ovariectomized (OVX) female rats were studied. Results were compared with control groups that did not receive T. In the absence of exogenous insulin, T treatment significantly suppressed phenylalanine release (an index of net protein degradation) from hindquarters of intact and OVX rats below that of the intact control group (P less than .01). In perfusions containing insulin (100 or 500 microU/mL), phenylalanine efflux from intact and OVX control hindquarters was significantly lower as compared with hindquarters perfused without exogenous insulin (P less than .01). Insulin also reduced the phenylalanine release by T-treated groups to the efflux rates of control rats perfused at the same insulin concentration. Basal glucose uptake by perfused hindquarters was similar in all four treatment groups. In perfusions containing 100 or 500 microU insulin/mL, glucose uptake by hindlimbs of intact and OVX control rats was increased 2.7-fold and 5-fold, respectively, above that observed in perfusions without insulin (P less than .01). T treatment did not alter glucose uptake by perfused hindquarters in the absence or presence of insulin under these experimental conditions. The results of these studies suggest that T has anabolic effects on skeletal muscle that are independent of exogenous insulin action.

Prospective studies of insulin sensitivity in normal women receiving oral contraceptive agents.

Seven normal premenopausal women were studied before (control) and after 3 and 6 months of oral contraceptive agent (OCA) administration (30 micrograms ethinyl estradiol plus 150 micrograms levonorgestrel). The plasma glucose responses during 75-g oral glucose tolerance tests were not altered by the OCA, but the 3- and 6-month plasma insulin responses significantly exceeded control values (P less than 0.05). On a separate morning a constant iv infusion of [3H]3-glucose was given throughout a 2-h basal period and during two successive 2-h euglycemic clamp procedures at iv insulin delivery rates of 10 and 40 mU/m2.min, respectively. Endogenous glucose production rates (milligrams per kg/min) were not altered after 3 or 6 months of OCA administration. Peripheral glucose utilization rates were expressed as M (milligrams per kg/min) or the ratio of M over the prevailing plasma insulin concentration (M/I). One or both parameters were significantly reduced below control values at both insulin infusion rates after 3 months (P less than 0.05), but returned toward control values after 6 months. Serum androgen concentrations were reduced or not altered by OCA administration. We conclude that insulin resistance induced by OCA administration is manifested by reduced peripheral tissue insulin sensitivity and may ameliorate with time. This effect does not relate consistently to total plasma insulin responses during oral glucose tolerance tests or to elevated serum androgen concentrations.

Relationship between neonatal birth weight and maternal plasma amino acid profiles in lean and obese nondiabetic women and in type I diabetic pregnant women.

Profiles of hemoglobin A1c (HbA1c) and concentrations of plasma glucose and 18 plasma amino acids were obtained in ten nonobese, insulin-dependent type I diabetic women, in 9 age- and weight-matched normal women and in ten obese nondiabetic women throughout pregnancy and postpartum. In late gestation, the period of maximum fetal growth, average HbA1c, plasma glucose, and total amino acid concentrations in diabetic mothers were significantly elevated above lean control values. No differences existed between the obese and lean control groups. Lean diabetic mothers also had significantly heavier babies (mean +/- SEM) relative to the 50th percentile for gestational age and sex (119 +/- 9%) than did the lean control group (94 +/- 3%, P less than .05). Relative birth weights among control lean and obese mothers did not differ significantly (94 +/- 3% v 104 +/- 5%). Late pregnancy profiles of HbA1c and average plasma glucose did not correlate with relative weight of neonates whereas average total plasma amino acids and six individual amino acids did correlate with this parameter. These data suggest that maternal plasma amino acid concentrations may influence fetal weight generally and may have an important role in the development of fetal macrosomia in diabetic pregnancies.

Morphometric studies of secretory granule distribution and association with microtubules in beta-cells of rat islets during glucose stimulation.

Morphometric studies of beta-cell granule positions and interactions with microtubules in areas of exocytosis were performed on islets of rats injected with tolbutamide for 72 h. This treatment depleted granules 85% below normal control values and rendered mapping of their positions and visualization of microtubules more feasible during electron microscopy. Islets were perifused with 2.8 or 16.7 mM glucose and were fixed at 4 min (acute phase) or 20 min (second phase). Photomicrographs of beta-cell sections were digitized, and computer replicas were compared to computer-simulated cells of similar size and shape into which an identical number of granules was inserted randomly. Margination of granules into a zone within 1.5 micron of the plasma membrane was examined. At both 2.8 and 16.7 mM glucose, real cells marginated significantly more secretory granules than did simulated cell pairs. Within the zones of margination during 16.7 mM glucose perifusion, the number of secretory granules associated with microtubules was less than 6% in the acute phase and less than 1% in the second phase. These data suggest that glucose stimulation promotes movement of secretory granules into the beta-cell periphery to a greater extent than can be attributed to chance alone. The role of microtubules in peripheral granule movement into regions of exocytosis appears to be minimal.

Fluctuations of alpha cell calcium, potassium and sodium during amino acid perifusion of rat pancreatic islets.

Perifusion of rat pancreatic islets with a physiologic, 6-mM amino acid mixture resulted in typical acute and second phase glucagon secretion over 30 min. At various intervals, islets were acutely fixed and processed for scanning electron microscopy, identification of alpha cells, and measurements of single alpha cell content of calcium (Ca), potassium (K) and sodium (Na) with energy-dispersive x-ray analysis. Biphasic glucagon secretion was attended by corresponding biphasic Ca accumulation and a reciprocal, biphasic suppression of K content and acute phase suppression of Na in alpha cells. All secretory and cellular events were preceded by an evanescent upward spike in alpha cell K at 1 min. These results indicate that alpha cell glucagon secretion in response to amino acid mixtures may be initiated by a K signal and is coupled subsequently to phasic changes in alpha cell Ca content. Fluctuations of alpha cell K and Na appear to relate inversely to Ca, suggesting that transmembrane fluxes of the three cations are interrelated.

Quantitative morphometric studies of pancreatic islets obtained from tolbutamide-treated rats.

We have developed a computerized system for quantitative morphometric analysis of the number and position of secretory granules and organelles in pancreatic islet beta cells following tolbutamide treatment. Data from animals injected with tolbutamide for 1, 2, and 3 days were compared to tissues obtained from untreated control animals. Pancreatic islets removed by a collagenase technique were perfused with an appropriate medium to restore a basal state. After fixation and embedding, thick sections of beta cells were viewed by electron microscopy. Morphometric studies of randomly selected or serially cut cells were performed with computer programs for digitization, quantify, rotational, and perspective display. Tolbutamide treatment resulted in graded granule depletion which was maximal at 72 hr relative to control animals. Reduced granule density was associated with significant reduction in total cell area or cytoplasmic area, but was without effect on nuclear size. Since granule depletion improved visualization of subcellular structures, this will enable us to pursue studies of exocytosis under a variety of physiological conditions.

Influence of endogenous opioids on the response of selected hormones to exercise in humans.

To examine the influence of endogenous opioids on the hormonal response to isotonic exercise, eight males were studied 2 h after oral administration of placebo or 50 mg naltrexone, a long-lasting opioid antagonist. Venous blood samples were obtained before, during, and after 30 min of bicycle exercise at 70% VO2max. Naltrexone had no effect on resting cardiovascular, endocrine, or serum variables. During exercise epinephrine was higher [mean 433 +/- 100 (SE) pg/ml] at 30 min with naltrexone than during placebo (207 +/- 26 pg/ml, P less than 0.05). Plasma norepinephrine showed the same trend but the difference (2,012 +/- 340 pg/ml with naltrexone and 1,562 +/- 241 pg/ml with placebo) was not significant. Plasma glucose was higher at all times with naltrexone. However, the difference was significant only 10 min into recovery from exercise (104.7 +/- 4.7 vs. 94.5 +/- 2.8 mg/dl). Plasma growth hormone and cortisol increased during recovery and these elevations were significantly (P less than 0.05) augmented by naltrexone. Plasma vasopressin and prolactin increased with exercise as did heart rate, blood pressure, lactic acid, and several serum components; these increases were not affected by naltrexone. Psychological tension or anxiety was lower after exercise compared with before and this improved psychological state was not influenced by the naltrexone treatment. These data suggest that exercise-induced activation of the endogenous opioid system may serve to regulate the secretion of several important hormones (i.e., epinephrine) during and after exercise.

Therapeutic results of insulin therapy in gestational diabetes mellitus.

Most studies of gestational diabetes mellitus (GDM) have reported a marked reduction in perinatal mortality with appropriate dietary regimens and good medical and obstetrical surveillance. Nevertheless, fetal morbidity, including macrosomia, has remained high and appears to be linked to factors other than plasma glucose control. In a review of six investigations in which insulin therapy was combined with an appropriate diet, the incidence of fetal macrosomia was reduced in five studies as compared with diet-only treatments. Again, the improvement did not always correlate with altered plasma glucose profiles. Other studies suggest that maternal plasma substrate disturbances other than glucose may contribute to the development of fetal macrosomia. To what extent insulin administration reduces morbidity by containing circulating maternal fuels, such as lipids and amino acids, in a more normal range remains to be determined. Moreover, the role of diet, maternal obesity, and weight gain during pregnancy adds to the complexity of factors influencing obstetrical outcome in gestational diabetes. Until the relative importance of all of these variables is adequately assessed, criteria for selection of women with pregnancy-onset diabetes for insulin therapy are most likely to be based on fasting and postprandial plasma glucose concentrations.

Assessment of methods for assigning treadmill exercise workloads for lean and obese women.

The use of relative oxygen consumption (% VO2 max) to equate workloads between trained and untrained subjects is considered appropriate. Whether this method (% VO2 max) is correct when testing lean versus obese subjects has not been studied. Using three experimental sessions separated by at least one week we studied seven obese, nondiabetic women weighting 140 +/- 21 kg (mean +/- s.d.) and 10 lean women of similar age weighing 51 +/- 5 kg during treadmill walking. Body fat determined by hydrostatic weighing was 50 +/- 7 and 23 +/- 7 percent in obese and lean groups, respectively (P less than 0.05). Absolute maximal VO2 (VO2 max) in obese women (3.18 +/- 0.28 l/min) significantly exceeded VO2 max for lean women (2.45 +/- 0.31 l/min, P less than 0.05). However, in obese women VO2 max relative to total body weight (TBW) was lower (P less than 0.05) while VO2 max relative to fat-free weight (FFW) was similar to corresponding values for lean women. Submaximal treadmill exercise at 75m/min with 0, 5 and 10 percent inclines resulted in higher VO2 (l/min) and heart rate (HR) in the obese group (P less than 0.05). At each incline VO2 relative to BW was significantly lower in the obese (P less than 0.05) yet significantly higher when expressed relative to FFW (P less than 0.05). The relationship between HR and %VO2 max was similar for lean and obese. Ten minutes of walking at 70% VO2 max resulted in no significant differences between the groups in the 10 min values for HR, total work done, rise in rectal temperature, plasma lactic acid, perceived exertion and oxygen consumption relative to FFW. We conclude that either fixed absolute workloads or fixed oxygen consumption (absolute or relative to BW or FFW) are inappropriate methods for equating workloads when lean and obese subjects are compared. Relative VO2 accurately defines physiologically comparable exercise intensities for these groups and this should be the method of preference when studies are designed to investigate metabolic, endocrine or cardiovascular similarities and/or differences between these groups.

A prospective study of body fat distribution and weight loss.

Previous studies have suggested an association between adipose tissue cellularity and body fat distribution, and between adipose tissue cellularity and ability to lose weight. To determine whether there was an association between body fat distribution and ability to lose weight, we prospectively studied 187 severely obese women. The women were all 50 percent or more above ideal body weight, with personal physician documentation of no known major illnesses. Weights were recorded at the beginning and end of a 3-week hospitalization and every 3 months following hospitalization, for up to 2 years. The ratio of waist girth to hip girth (WHR) was used as an index of body fat distribution. A statistical analysis which adjusted for age and weight on admission did not find any association between WHR and weight loss during hospitalization, or at any time up to 2 years after hospital discharge. We conclude that the WHR index of body fat distribution is not a useful prognostic indicator of weight change for severely obese women with refractory obesity.

Relationship of body fat topography to insulin sensitivity and metabolic profiles in premenopausal women.

The relationship of body fat distribution to metabolic profiles was determined in 80 healthy premenopausal white women of a wide range of obesity levels [percentage of ideal body weight (% IBW) 92-251]. Distribution of fat between the upper and lower body was assessed from the waist/hips girth ratio (WHR), which varied from 0.64 to 1.02. In 23 women, in vivo insulin sensitivity was also determined from the steady-state plasma glucose (SSPG) level at comparable insulin levels of approximately 100 microU/mL attained by the intravenous infusion of somatostatin, glucose, and insulin. Increasing WHR was accompanied by progressively increasing fasting plasma insulin levels (r = 0.47, P less than 0.001), insulin and glucose areas after glucose challenge (r = 0.53, P less than 0.001; r = 0.50, P less than 0.001, respectively) and fasting plasma triglyceride concentrations (r = 0.48, P less than 0.001). Obesity level was similarly correlated with these metabolic indices. Partial and multiple regression analysis and analysis of variance with a linear contrast model revealed that the effects of body fat topography were independent of, and additive to, those of obesity level. Within obese subjects alone (%IBW: 130), %IBW had no predictive value, but WHR remained a significant predictor of plasma glucose, insulin, and triglyceride concentrations. The WHR also correlated with the plasma cholesterol level, but this association was largely dependent on its relationship to %IBW. Both WHR and %IBW correlated with the insulin resistance index, SSPG (r = 0.60, P less than 0.01; r = 0.61, P less than 0.01, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship of body fat distribution to blood pressure, carbohydrate tolerance, and plasma lipids in healthy obese women.

In 110 obese, healthy women, a relationship was sought between distribution of body fat and blood pressure, glucose tolerance, plasma insulin, and fasting plasma lipid and serum uric acid concentrations. The index of body fat distribution was the ratio of waist circumference to hips circumference (WHR). The WHR range in this group was 0.5 to 0.99, with a median value of 0.78. Positive, significant correlations were found between WHR and both systolic and diastolic blood pressure and between WHR and the total integrated plasma glucose and insulin responses during 4 hr oral glucose tolerance tests. No relationship was found between WHR and age, the degree of obesity as defined by the weight-to-height ratio, or concentrations of fasting plasma free fatty acids, plasma triglyceride, plasma cholesterol, or serum uric acid. Subsequently, 27 women in the highest quartile of the WHR range (0.83 to 0.99) were compared to 28 age- and weight-matched subjects in the lowest quartile of WHR (0.5 to 0.73). Women in the highest quartile had systolic and diastolic blood pressure as well as total plasma glucose and insulin responses during glucose tolerance tests that significantly exceeded mean values of subjects in the lowest quartile. We conclude that in healthy, obese women, a continuum exists that relates increasing fat accumulation in the upper body to progressively higher blood pressure, reduced carbohydrate tolerance, and higher plasma insulin concentrations. These changes occurred independently of age of degree of obesity in this population.

Tolbutamide perifusion of rat islets. Sequential changes in calcium, phosphorus, sodium, potassium, and chlorine in single beta cells.

Fluctuations of calcium, phosphorus, sodium, potassium, and chlorine in beta cells were followed during rat islet perifusion with tolbutamide and related to insulin secretion. In 24 paired experiments two chambers containing 100 islets were perifused with buffered medium containing 4.2 mM glucose alone or with added tolbutamide (200 micrograms/ml). Effluent was collected frequently for insulin determinations. At eight different time intervals from 0 to 20 min islets were acutely fixed, prepared for scanning electron microscopy and beta cells in islet tissue were identified. Element content in 480 single cells was measured by energy dispersive x-ray analysis. Tolbutamide elicited typical monophasic insulin release that exceeded control islet secretory rates from 2 to 6 min with a peak value at 3 min. This pattern was preceded by monophasic calcium accumulation in beta cells that abruptly rose 150% above control cells at 1 min and declined to base line by 4 min. The rapid ascent of calcium was associated with significant depressions of sodium and potassium content without alterations of cell phosphorus. Chlorine fell at 2 min and then rose greater than 50% above control cells at 4 min. After 6 min insulin secretion and element content remained near control levels. We conclude that monophasic calcium accumulation in beta cells is the earliest, most predictive event of islet insulin secretion after a tolbutamide stimulus. Oscillations of beta cell sodium and potassium reciprocally relate to calcium, and an elevation of chlorine content is a relatively late phenomenon in the stimulus-secretion coupling process.