Ultrastructural detection of granulated cells in the autonomic ganglia of the rat pancreas

Expression of voltage-gated potassium channels in human and rhesus pancreatic islets

Voltage-gated potassium channels (Kv channels) are involved in repolarization of excitable cells. In pancreatic beta-cells, prolongation of the action potential by block of delayed rectifier potassium channels would be expected to increase intracellular free calcium and to promote insulin release in a glucose-dependent manner. However, the specific Kv channel subtypes responsible for repolarization in beta-cells, most importantly in humans, are not completely resolved. In this study, we have investigated the expression of 26 subtypes from Kv subfamilies in human islet mRNA. The results of the RT-PCR analysis were extended by in situ hybridization and/or immunohistochemical analysis on sections from human or Rhesus pancreas. Cell-specific markers were used to show that Kv2.1, Kv3.2, Kv6.2, and Kv9.3 are expressed in beta-cells, that Kv3.1 and Kv6.1 are expressed in alpha-cells, and that Kv2.2 is expressed in delta-cells. This study suggests that more than one Kv channel subtype might contribute to the beta-cell delayed rectifier current and that this current could be formed by heterotetramers of active and silent subunits.

An immunocytochemical study of intrapancreatic ganglia, nerve fibres and neuroglandular junctions in Brockmann bodies of the tompot blenny (Blennius gattoruggine), a marine teleost

The innervation of the Brockmann bodies in the teleost fish, Blennius gattoruggine, was studied using immunocytochemical techniques at both the light and electron microscopy levels. Islet innervation consisted of intrapancreatic ganglia, generally localized inside the rim of the exocrine tissue of the Brockmann bodies, in proximity to the islet, nerve fibres and nerve terminals with synaptic complexes. The intrapancreatic ganglia were of variable size, with different numbers of ganglionic cells, that appeared unipolar in section. The cell bodies showed immunoreactivity to galanin, oxytocin, peptide tyrosine tyrosine and glucagon. The extrinsic and intrinsic nerve fibres passed through the exocrine parenchyma and crossed the connectival septa and islet connectival sheath, penetrating into the islets, where they became increasingly thinner. They terminated on the endocrine cells with dilated nerve terminals. At least three types of terminals were detected, depending on the different vesicle content: peptidergic, cholinergic or adrenergic. They presented specialized synaptic structures, the neuroglandular junctions, some of which contained neurosecretory granules immunogold labelled by galanin antiserum. This new finding confirms the role of galanin as a neurotransmitter. This rich supply of innervation may be important in the regulation and integration of islet secretion.

Increased disorderliness of basal insulin release, attenuated insulin secretory burst mass, and reduced ultradian rhythmicity of insulin secretion in older individuals

Insulin is secreted in a pulsatile fashion. Rapid pulses are considered to be important for inhibiting hepatic glucose output, and ultradian pulses for stimulating peripheral glucose disposal. Aging is characterized by a progressive impairment in carbohydrate tolerance. We undertook the current studies to determine whether alterations in pulsatile insulin release accompany the age-related changes in carbohydrate metabolism. Healthy young (n = 8; body mass index, 21 +/- 1 kg/m2; age, 24 +/- 1 yr) and old (n = 9; body mass index, 24 +/- 1 kg/m2; age, 77 +/- 2 yr) volunteers underwent two studies. In the first study, insulin was sampled every 1 min for 150 min, and pulse analysis was conducted using a recently validated multiparameter deconvolution technique. In the second study, insulin was sampled every 10 min for 600 min, and insulin release was evaluated by Cluster analysis. In the 150-min studies, insulin secretory burst mass (P < 0.05) and amplitude (P < 0.01) were reduced in the elderly. In addition, approximate entropy, a measure of irregularity or disorderliness of insulin release, was increased in the aged (P < 0.01). In the 600-min studies, interpulse interval was greater in the aged (P < 0.05), and burst number was less (P < 0.05). We conclude that normal aging is characterized by more disorderly insulin release, a reduction in the amplitude and mass of rapid insulin pulses, and a decreased frequency of ultradian pulses. Whether these alterations in insulin pulsatility contribute directly to the age-related changes in carbohydrate metabolism will require further study.

Adrenoceptor antagonists, but not guanethidine, reduce glucopenia-induced glucagon secretion from perfused rat pancreas

This study was designed to investigate (1) whether norepinephrine is released in response to glucopenia in vitro, thereby stimulating glucagon secretion and, (2) the modulating effects of norepinephrine on insulin and glucagon secretion, using isolated perfused rat pancreas preparations. Simultaneous addition of the adrenergic receptor antagonists yohimbine, prazosin and propranolol, each at a concentration of 10-(5) mol/l, significantly potentiated glucose-stimulated insulin secretion (6.23 +/- 0.76 vs. 2.11 +/- 0.72 (control) nmol/min, P < 0.01), and suppressed glucopenia-induced glucagon secretion (0.59 +/- 0.10 vs. 1.34 + 0.18 (control) ng/min, P < 0.05). Also, 10-(5) mol/l yohimbine alone significantly potentiated glucose-stimulated insulin secretion (4.86 +/- 0.50 nmol/min, P < 0.05). The norepinephrine release inhibitor, guanethidine, significantly inhibited tyramine-induced secretion of both norepinephrine (7.86 +/- 0.77 vs. 49.7 +/- 2.3 nmol/min, P < 0.01) and glucagon (0.31 +/- 0.08 vs. 1.21 +/- 0.15 ng/min, P < 0.01), but exerted no effects on glucopenia-induced secretion of either norepinephrine or glucagon. We conclude that these results further support the concept that the neurotransmitter norepinephrine is released in response to glucopenia in vitro, and modulates insulin and glucagon secretion. Our data do not, however, provide evidence indicating that glucopenia-induced glucagon secretion is mainly mediated by activation of sympathetic nerve terminals around the alpha-cells in the isolated perfused rat pancreas.

Neurotransmitters partially restore glucose sensitivity of insulin and glucagon secretion from perfused streptozotocin-induced diabetic rat pancreas

To elucidate the mechanisms of insensitivity of hormone secretion to glucose in streptozotocin-induced diabetic rat islets, we investigated the effects of acetylcholine (ACh) and norepinephrine on insulin and glucagon secretion in response to changes in glucose concentration, using perfused pancreas preparations. Basal insulin secretion at a blood glucose level of 5.6 mmol/l was significantly higher and basal glucagon secretion significantly lower in streptozotocin-induced diabetic rats than in controls, and neither high (16.7 mmol/l) nor low (1.4 mmol/l) blood glucose concentrations influenced insulin or glucagon secretion. Addition of 10(-6) mol/l ACh to the perfusate increased glucose-stimulated insulin secretion. Also, 10(-6) mol/l ACh, 10(-7) mol/l norepinephrine, as well as a combination of both, induced marked glucagon secretion, this was suppressed by high blood glucose level. Although simultaneous addition of 10(-6) mol/l ACh and 10(-7) mol/l norepinephrine induced only a slight increase in glucagon secretion in response to glucopenia, there was a significant increase in glucagon secretion in conjunction with an ambient decrease in insulin. Histopathological examination revealed a marked decline in acetylcholinesterase and monoamine-oxidase activities in the islets of streptozotocin-induced diabetic rats. We speculate that reduction of the potentiating effects of ACh and norepinephrine lessens glucose sensitivity of islet beta and alpha cells in this rat model of diabetes.

Comparison of insulin and glucagon pulsatile secretion between the rat and dog pancreas in-vitro

Sustained pulses of insulin and glucagon were obtained from the isolated perfused in vitro rat pancreas. The respective periodicity of hormone release (peak to peak interval) was calculated by the Pulsar computer algorithm as insulin 5.8 +/- 0.3 min and glucagon 6.5 +/- 0.25 min. Because pulsatile insulin secretion is absent in type II diabetics, pulsatile islet hormone secretion could theoretically be regulated directly by intra-islet hormone interactions or indirectly by hormone sensitive nerve feedback, possibly from a venous hormone sensitive receptor system within the pancreas. To test the possible contributions of these systems in pulse regulation, the direction of perfusion was reversed in both rat and dog pancreata to prevent hormone contact with putative venous hormone receptors. The periodicity of hormone secretion was unchanged by reversed perfusion in both species. As vascular perfusion of islet cells is normally B to A to D, these results suggest that neither intra-islet hormone interactions nor intra-pancreatic insulin or glucagon sensitive nerve feedback systems are responsible, on an acute basis, for the regulation of pulsatile insular secretion from the normal pancreas. Insulin regulates net glucagon secretion but does not acutely influence glucagon pulses. The presence of pulses during retrograde perfusion may be the result of the entrainment of the pacemaker-islet system. These observations are consistent with the presence of an independent pacemaker and neural coordinating system within the dog and rat pancreas which may influence both the A- and B-cell.

Lack of direct inhibition of insulin secretion by exogenous insulin in the canine pancreas

To test whether insulin secretion is self-regulatory, canine pancreata were isolated and perfused in vitro and were infused with 0.3, 0.6, or 1.2 mU/ml exogenous insulin. Basal and arginine-stimulated concentrations of C-peptide, glucagon, and somatostatin were measured. There were no significant differences between basal secretion nor the increment of arginine-stimulated secretion for each respective hormone at each exogenous insulin concentration. The second preparation studied was a vascularly isolated, yet innervated, in situ perfused pancreas. Exogenous insulin (1 mU/kg per min) was infused "systemically"; the pancreas received no insulin. Endogenous pancreatic insulin and C-peptide secretion was suppressed, while pancreatic glucagon secretion increased during systemic insulin infusion. No changes in pancreatic hormone secretion occurred after the sympathetic nerves were sectioned. These results suggest that exogenous insulin does not directly suppress the B cell, but can suppress insulin secretion through an indirect neurally mediated, insulin-dependent nerve mechanism.