Portal vein insulin flux and arterial glucose fluctuations in response to an oral meal test in islet cell-transplanted dogs.

Insulin flux was determined in the portal vein and simultaneously arterial blood glucose was measured before and during an oral glucose meal in conscious normal and pancreatic islet cell-autotransplanted dogs to test their insulinogenic reserve. These dogs had previously been chronically instrumented with blood flow probes on the portal vein and carotid artery, and blood sampling catheters in the portal vein, hepatic vein, carotid artery, and right external jugular vein. Such a model permits quantitative portal-peripheral comparisons and assessment of hepatic extraction. Sixteen dogs, 10 normal (N) and six long-term (2 months to 2 yrs) islet cell-transplanted dogs (IT) were fed an oral glucose meal as a test (OMT). Baseline portal vein insulin fluxes (PVF) were similar in both groups (25.6 +/- 0.04 pmol/min in N and 24.7 +/- 19.4 pmol/min in IT). Immediately after OMT, PVF rose to 248.2 +/- 40.9 pmol/min in N, but only to 55.9 +/- 17.9 pmol/min in IT. After 30 min PVF peaked for the second time in N at 156 +/- 35.9 pmol/min, declining slowly to baseline after 3 h. In IT, a similar peak at 30 min was seen (143.7 +/- 22.1 pmol/min), declining to a value not different from baseline after 3 h. However, cumulative insulin PV fluxes in the two groups over 3 h were not different. Differences were also seen in postprandial glucose fluctuations, which reached a maximum excursion of 11.8 +/- 0.45 mM in IT, while never rising above 7.8 +/- 0.33 mM in N. After 3 h both groups had similar glucose values.(ABSTRACT TRUNCATED AT 250 WORDS)

A chronic conscious dog model for direct transhepatic studies in normal and pancreatic islet cell transplanted dogs.

A chronic conscious, large mammal model for repeated transhepatic studies over a period of 6-8 weeks has been developed for application to the study of the hepatic effects of pancreatic hormone secretion and glucose metabolism, studies of the hepatic mechanisms associated with high first-pass drug metabolism and food interactions, and studies of insulin balance in dogs that have undergone previous pancreatectomy and islet cell auto-transplantation. The preparation of specialized blood sampling catheters and blood flow probes, surgical preparations, pre- and postoperative care, catheter maintenance, and possible complications are described. Data from an oral glucose meal (OMT) are presented. Frequent blood samples from portal, hepatic and jugular veins, and carotid artery were collected and analyzed for plasma insulin concentrations (IRI) and glucose. IRI fluxes were determined for each time period by interpolation of flux X time curves. Total IRI flux was determined as the sum of the areas under the curves for each sampling period. Hepatic insulin extraction was calculated for each sampling interval. By this method of analysis, it was possible to determine hepatic IRI extraction during non-steady-state conditions. The techniques described in the development of this complex animal model, which allows for repeated transhepatic studies in the same conscious subject, may also have application in other chronic studies involving organs other than the liver.

The effect of somatostatin on insulin and glucose levels during insulin infusion in anesthetized dogs.

The purpose of the study was to examine the transhepatic and peripheral effects of somatostatin (SN) infusion on plasma glucose and insulin during insulin (IN) infusion. Hepatic blood flow was measured electromagnetically during intermittent sampling from the portal and hepatic veins, femoral artery, and right external jugular vein. Hepatic blood flow [sum of portal vein (PV) and hepatic artery] was similar during IN or IN + SN infusions. IN concentrations decreased in the portal vein from 374.8 +/- 50.3 to 295.8 +/- 25.9 pM (p less than 0.01) when SN was infused with IN. Hepatic venous plasma IN concentration also decreased from 143.6 +/- 26.6 to 88.3 +/- 10.1 pM (p less than 0.01). Plasma IN concentrations in the femoral artery and jugular vein remained unchanged. Hepatic insulin extraction changed from 64 +/- 4% during IN to 72 +/- 3% during IN + SN (p less than 0.01). Hepatic clearance and total body clearance were unchanged. Peripheral venous glucose with a nadir of 3.82 +/- 0.2 mM during IN alone decreased to a nadir of 3.16 +/- 0.27 mM (p less than 0.01) during IN + SN infusion. Mean portal venous glucose concentrations were 5.0 +/- 0.27 and 3.4 +/- 0.19 mM, respectively (p less than 0.01). In two additional experiments in which endogenous C-peptide concentrations were examined in the portal vein and femoral artery, C-peptide levels were lower during IN + SN compared to IN alone. We conclude that SN used to suppress endogenous insulin secretion increases hepatic insulin extraction, lowers glucose concentrations, and suppresses endogenous C-peptide levels to a greater extent than insulin infusion alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Somatostatin increases hepatic insulin extraction.

Somatostatin has been widely employed in studies of hepatic metabolism to suppress the endogenous secretion of the pancreatic hormones, insulin and glucagon. The possibility of somatostatin having hepatic effects has been considered before, but not decisively demonstrated. Eleven anesthetized dogs were used to assess the effect of somatostatin on hepatic insulin extraction (HIE) and hepatic glucose production (HGP). Insulin was infused by peripheral vein alone and during somatostatin (800 ng/kg/hr) in one series (I) of experiments and in the reverse sequence in another series (II). Portal vein and hepatic artery blood flow were measured electromagnetically, and blood samples were taken from the hepatic artery, portal vein, hepatic vein and a peripheral vein. HIE was increased in the presence of insulin plus somatostatin infusion. The combined results in Series I and II were 72.0 +/- 3.0% compared in insulin alone (64.0 +/- 4.0%, p less than 0.01). HGP was decreased in the presence of insulin and somatostatin infusion compared to insulin infusion alone both in Series I and II (1.71 +/- 0.25 to 3.72 +/- 4.0%, combined results, p less than 0.01). Whether this reduction in HGP indicates a direct effect of somatostatin on the liver, in addition to the inhibition of glucagon secretion, remains to be clarified. However, a decrease in hepatic glucose production is consistent with increased insulin extraction during somatostatin observed in the present study. We conclude that somatostatin increases the hepatic extraction of exogenous insulin. This effect of somatostatin is associated with decreased hepatic glucose production.

Accuracy of C-peptide:insulin molar ratio as a measure of hepatic removal of insulin.

We measured transhepatic C-peptide and insulin concentrations in plasma, and hepatic removal of insulin, to examine whether the practice of reporting the C-peptide:insulin molar ratio as a measure of the hepatic removal of insulin is valid. In anesthetized dogs (n = 6), during electromagnetic hepatic blood flow monitoring, endogenous insulin was suppressed with somatostatin, while equimolar proportions of porcine insulin and simian C-peptide (2.4 and 6.0 pmol/kg.min) were infused during two consecutive 45-min periods. Insulin reached steady state within 20 min (t1/2 = 4.5 min); however, C-peptide concentrations continued to rise (t1/2 V 12.5 min). The ratio decreased when the peptide infusion was changed to the higher rate and increased when it was stopped, reflecting the more rapid removal of insulin than of C-peptide. Hepatic removal of insulin remained constant during the two infusion periods (average 60% extraction) and never correlated with the changing molar ratios. Hepatic net flux of insulin correlated with the ratio (P less than 0.05) only while plasma insulin concentrations were rising during constant-rate infusion. We therefore conclude that the molar ratio is not a reliable measure of the hepatic removal of insulin during non-steady states of insulin or C-peptide.

Pharmacokinetic approach to the estimation of hepatic removal of insulin.

We simultaneously measured hepatic insulin removal invasively and estimated hepatic clearance and extraction of insulin pharmacokinetically from cardiac output and peripheral plasma concentrations (relatively) noninvasively. The invasive methods involved continuous electromagnetic measurements of portal venous and hepatic arterial blood flow and simultaneous intermittent sampling of blood from the portal and hepatic veins and femoral artery for assay of insulin concentrations. The noninvasive method assumed that hepatic plasma flow is proportional to cardiac output and that hepatic clearance is a constant fraction of total body clearance of insulin. In anesthetized dogs (n = 6), endogenous insulin was suppressed with somatostatin (800 ng/kg/min) while biosynthetic human insulin (0.25, 0.50, and 1.00 mU/kg/min) was infused to steady state during three consecutive 90-min periods. Insulin concentrations were directly proportional to the infusion rate (p less than 0.01). Hepatic blood flow accounted for 20 +/- 2% of cardiac output. Measured hepatic clearance accounted for 51 +/- 5% of total body clearance of insulin and correlated with the pharmacokinetic estimates (p less than 0.01); the estimates of hepatic clearance ranged from 91 to 114% of the measured values. We conclude that this pharmacokinetic approach, which requires only samples of peripheral blood and estimates of hepatic blood flow, may be used to study the hepatic removal of insulin relatively noninvasively.

The effect of hepatic insulin influx on insulin retention: comparison of net flux and per cent extraction as indices of hepatic insulin retention.

We studied effects of changes in hepatic insulin influx on hepatic insulin retention. In order to compare insulin net flux (influx - efflux) with percentage (%) hepatic insulin extraction (net flux/influx X 100), as indices of hepatic insulin retention, we examined which index is better predicted by insulin influx to the liver. Electromagnetic hepatic blood flow measurements were made and blood was sampled from portal, hepatic and peripheral veins and femoral artery in anesthetized dogs. Insulin influx was perturbed by sequentially superimposed peripheral venous infusions of somatostatin, glucagon and insulin (which were subsequently discontinued one at a time). Although the somatostatin was biologically effective in inhibiting insulin and glucagon release we found no significant plasma flow changes on starting or stopping the infusion of somatostatin (800 ng X kg-1 X min-1). Net insulin flux was linearly related to influx (n = 56, r = 0.99, p less than 0.001). Hepatic glucose production was better correlated with net insulin flux (r = -0.66; p less than 0.01) than with % extraction (r = 0.08; N.S.), when mean glucagon influx was held constant (partial correlation). Although net insulin flux and % extraction are complementary indices, the former reflects influx changes more faithfully than the latter.

Hepatic insulin retention depends on influx in both diabetic and normal dogs.

Hepatic retention of endogenous and exogenous insulin was investigated in normal and alloxanstreptozotocin diabetic dogs. An hour's peripheral venous infusion of insulin was preceded and followed by saline infusion. Blood samples were drawn simultaneously from the femoral artery and portal, hepatic and peripheral veins. Blood flow was measured electromagnetically in the portal vein and hepatic artery. Hepatic plasma flow was lower in diabetic dogs (p less than 0.001) so that prior to insulin infusion (basal hour) basal mean hepatic venous flux was diminished in them. Hepatic glucose production was higher (p less than 0.05) and arterio-venous glucose difference lower (p less than 0.01) in diabetics basally, but post-insulin infusion, these became similar in diabetics and normals. Although basal hour arterial and peripheral venous insulin concentrations were similar, portal venous insulin was 3.5-fold lower in diabetics. Mean insulin influx (hepatic arterial + portal venous fluxes) was four-fold lower in diabetics. The insulin net flux (influx - hepatic venous flux) was lower in diabetics (p less than 0.05) as was % extraction (p less than 0.001). During the insulin infusion hour the values remained lower for diabetics (p less than 0.05) for influx, net flux and % extraction. During the post-insulin infusion hour the influx, net flux and % extraction became similar. When the basal hour data in normals was restricted to the range of diabetic insulin influx values less than or equal to 10 mU X min-1, the differences in net flux and % extraction between normals and diabetics were no longer present.(ABSTRACT TRUNCATED AT 250 WORDS)

Methods of assessing diabetic control.

Control of diabetes from complete normalisation to less adequate degrees of metabolic regulation needs to be assessed with regard to conditions of evaluation and to severity of the disease. Under optimal conditions the therapeutic events should occur with well-timed regularity. Different assessment criteria are appropriate depending on the severity of the deficiency of endogenous insulin. Plasma and urine glucose and ketone body measurements remain the practical standards for assessing diabetic control. Abnormalities of lipid and protein metabolites serve to augment the scope of the assessment. Triglycerides and haemoglobin AIc are also useful indicators of control. In mild (Type II) diabetes it may be possible to achieve normal plasma glucose measurements two hours after meals. Such aims carry a risk of hypoglycaemia in severe (Type I) diabetes. Normoglycaemia and aglycosuria in severe diabetes are feasible only preprandially in most cases. The use of urine glucose tests requires evaluation of blood-to-urine glucose relationships. Practical and convenient methods for identifying patients with high or low "renal thresholds" are described. Investigational methods for characterising diabetic patients assess the variability of glucose and other variables during therapy, as well as the degree to which normal values are attained. Such assessment methods may gain increasing practical importance as therapeutic approaches to diabetic control which are experimental at present come into practice.

C-peptide suppression test for insulinoma.

During hypoglycemia induced by an infusion of porcine insulin, impaired suppression of endogenous insulin secretion as measured by C-peptide was demonstrated in 11 of 12 patients with insulinoma. During hypoglycemia (plasma glucose less than or equal to 40 mg/dl) the mean C-peptide immunoreactivity (CPR) of normal subjects was less than or equal to 1.2 ng/ml, whereas 11 of 12 insulinoma patients had a mean CPR of larger than or equal to 1.9 ng/ml. One patient showed normal CPR suppression by these criteria but may have shown impaired CPR suppression for glucose less than or equal to 30 mg/dl. Impaired CPR suppression during insulin-induced hypoglycemia may prove to be a useful test for insulinoma.

Prolonged follow-up in diabetic retinopathy treated by sectioning the pituitary stalk.

Because no complete long-term follow-up of visual status after pituitary ablation has been reported, records were reviewed on all 40 patients undergoing sectioning of the pituitary stalk for diabetic retinopathy at the Mayo Clinic from 1961 through 1968. One patient died and another was blind immediately postoperatively. One patient was lost to follow-up after 125 months. Twenty-seven patients have died after a follow-up interval of 7 to 120 months (mean, 63 months). The last available evaluation of these patients indicated stable or improved visual acuity in 20 patients. Eleven patients remain alive. Seven of these have stable or improved visual acuity and retinopathy. We conclude that although late post-operative mortality is high, in carefully selected patients with florid retinopathy but no fibrosis, pituitary ablation is an effective method for maintaining visual acuity.

Abnormalities of endogenous glucagon and insulin in unstable diabetes.

The responses of glucagon, growth hormone, and insulin secretion to the oral administration of glucose and to the intravenous infusion of saline, arginine, and insulin were measured in seven patients who had stable diabetes, eight who had unstable diabetes, and seven healthy volunteers. Hyperglycemia suppressed secretion of glucagon in normal subjects but not in diabetics. The oral glucose and arginine infusion tests demonstrated partial preservation of insulin-secretory ability in stable diabetics and its virxual absence in unstable diabetics. Glucagon responses to arginine infusion were similar in all three groups. In response to hypoglycemia induced by insulin infusion, the concentrations of plasma glucagon increased in normal subjects and, to a lesser extent, in stable diabetics but increased in only two of the unstable diabetics. The impairment in glucagon response during hypoglycemia in diabetics correlated positively with the degree of diabetic instability and insulin deficiency during glucose and arginine testing. The severity of the insulin deficiency also correlated with the degree of diabetic instability. These findings support the hypothesis that inherent abnormalities of insulin and glucagon secretion may account for many of the clinical characteristics of unstable and stable diabetic patients.

Prolonged suppression of insulin release by insulin-induced hypoglycemia: demonstration by C-peptide assay.

Pancreatic beta cells secrete the proinsulin connecting peptide (C-peptide) and insulin on an equimolar basis. The C-peptide can thus be used as an indicator of endogenous insulin secretion in the presence of exogenously administered insulin. Using this approach, we have shown suppression of endogenous insulin release in healthy subjects during hypoglycemia induced by intravenous infusion of porcine insulin. Moreover, the suppression persists after the plasma glucose returns to fasting levels, suggesting that the recovery of beta cells from the effects of hypoglycemia is not immediate.