Calcium induced glucagon release in monolayer culture of the endocrine pancreas. Studies with ionophore A23187

Distribution Profile of Inositol 1,4,5-Trisphosphate Receptor/Ca2+ Channels in α and β Cells of Pancreas: Dominant Localization in Secretory Granules and Common Error in Identification of Secretory Granule Membranes

OBJECTIVES

The α and β cells of pancreatic islet release important hormones in response to intracellular Ca increases that result from Ca releases through the inositol 1,4,5-trisphoshate receptor (IP3R)/Ca channels. Yet no systematic studies on distribution of IP3R/Ca channels have been done, prompting us to investigate the distribution of all 3 IP3R isoforms.

METHODS

Immunogold electron microscopy was performed to determine the presence and the relative concentrations of all 3 IP3R isoforms in 2 major organelles secretory granules (SGs) and the endoplasmic reticulum of α and β cells of rat pancreas.

RESULTS

All 3 IP3R isoforms were present in SG membranes of both cells, and the IP3R concentrations in SGs were ∼2-fold higher than those in the endoplasmic reticulum. Moreover, large halos shown in the electron microscope images of insulin-containing SGs of β cells were gap spaces that resulted from separation of granule membranes from the surrounding cytoplasm.

CONCLUSIONS

These results strongly suggest the important roles of SGs in IP3-induced, Ca-dependent regulatory secretory pathway in pancreas. Moreover, the accurate location of SG membranes of β cells was further confirmed by the location of another integral membrane protein synaptotagmin V and of membrane phospholipid PI(4,5)P2.

Stimulation by calcium and barium of somatostatin release. Evidence for lower sensitivity of D- vis-à-vis B- and A-cells

To address the question whether a "second messenger" function of calcium differs between D-cells and other cells of the endocrine pancreas, we compared effects of calcium and barium (a calcium substitute) on somatostatin secretion to effects on insulin and glucagon secretion from the perfused pancreas of the rat. 6.5 mmol/l of calcium, when administered early during perfusion, failed to stimulate somatostatin release. 0.05 mmol/l of barium, when added to calcium-deprived media failed to affect somatostatin secretion while 0.5 induced a slight and 2.0 mmol/l a marked and sustained response. Barium-induced insulin release was left-shifted in relation to the somatostatin response, since 0.05 mmol/l of barium stimulated and 0.5 mmol/l evoked a near-maximal insulin response. All concentrations of barium evoked diphasic glucagon responses, i.e. a small (1 min) stimulation followed by sustained inhibition. Addition of 0.5 mmol/l of EGTA to calcium-deprived media abolished D- as well as B- and A-cell secretion. Reintroduction of 0.5-6.5 mmol/l of calcium stimulated somatostatin release; the secretory response was proportionate to the calcium concentration. In contrast, addition of calcium stimulated insulin and glucagon secretion maximally already at 0.5 mmol/l of calcium. We conclude that the D-cell is less sensitive than B- and A-cells to a regulatory effect on secretion exerted by extracellular calcium or barium.

Glucagon and insulin secretion during acid-base alterations

Previously, we reported that change from the normal pH of 7.4 surrounding the islet cells to 7.8 results in a decreased B-cell response to 16.6 mM glucose, 10 mM arginine or 400 micrograms/ml tolbutamide. In the present report we studied the effect of modifications in the extracellular pH on glucose- and arginine-induced glucagon and insulin secretion by the perfused rat pancreas. It was found that at pH 7.8, arginine-induced glucagon secretion was significantly greater than at pH 7.4. On the other hand, the switch from pH 7.4 to 7.8 in a pancreas already stimulated by either low glucose or arginine, produced fast and transient glucagon release. Sequential extracellular pH changes from 7.4 to 7.8 and back to 7.4 in the presence of 8.3 mM glucose and a 5 mM arginine stimulus demonstrated that A- and B-cells rapidly modify their secretion in response to extracellular alkalosis in opposite directions. While glucagon output was enhanced (mean secretory rates at pH 7.4, 0.692 +/- 0.042 ng/min and 0.948 +/- 0.57 at pH 7.8), insulin secretion was clearly reduced (72.6 +/- 6.2 ng/min and 35.7 +/- 2.8 ng/min at pH 7.4 and 7.8, respectively). The above observations, together with our previously reported data, indicate that extracellular pH plays an important role in the regulation of glucagon and insulin release. Particularly, extracellular alkalosis enhances A-cell response to 2.3 mM glucose and 5 mM arginine while partially inhibiting B-cell secretion in the perfused rat pancreas.

Difference in calcium dependency of insulin, glucagon and somatostatin secretion in response to glibenclamide in perfused rat pancreas

The extracellular calcium requirements for insulin, glucagon and somatostatin release induced by 1 microgram/ml of glibenclamide have been compared in the perfused, isolated rat pancreas. In the absence of glucose, the drug evoked insulin release equally well at physiological (2.6 mmol/l) and low (0.25 mmol/l) levels of total calcium. In contrast, glibenclamide evoked somatostatin release at 2.6 but not at 0.25 mmol/l of calcium. At 2.6 mmol/l of calcium, glibenclamide evoked bimodal effects (stimulation followed by inhibition) on glucagon secretion. At 0.25 mmol/l of calcium, basal secretory rates of glucagon were elevated and a small stimulatory effect of glibenclamide was seen. Addition of 0.5 mmol/l of EGTA to media with low calcium concentrations uniformly abolished the A, B and D cell secretory responses to glibenclamide. The possible modulation of calcium dependency by a non-stimulatory concentration of glucose was tested by its addition at 3.3 mmol/l to the perfusion media. Glucose enhanced glibenclamide-induced insulin secretion, both at 0.25 and 2.6 mmol/l of calcium. However, at 0.25 mmol/l of calcium, the enhancing effect of glucose was more pronounced than at 2.6 mmol/l. At 2.6 mmol/l of calcium, glucose diminished the somatostatin and abolished the glucagon response to glibenclamide. At 0.25 mmol/l of calcium, glucose did not influence somatostatin release while the presence of the sugar diminished basal and glibenclamide-induced glucagon secretion. The present data confirm the requirement of extracellular calcium for A, B and D cell secretion, demonstrating different calcium dependencies for the cell types and indicate that this dependency can, in part, be modulated by glucose.

Stimulation of renin secretion from the intact kidney and from isolated glomeruli by the calcium ionophore A23187

The ionophore A23187 evoked a dose-dependent release of renin from the isolated perfused cat kidney, which was inhibited by calcium deprivation and adrenergic blockade. The latter finding indicates that the effects of A23187 on the intact kidney are mediated mainly by catecholamine release from sympathetic nerve endings. Ionophore also elicited a concentration-dependent enhancement of renin secretion from a pure preparation of glomeruli isolated from cat kidney; this stimulation was still manifest when the glomeruli were superfused with a calcium-free solution. These findings indicate that A23187 evokes renin secretion from juxtaglomerular cells by mobilizing cellular calcium and support the view that an increase in intracellular calcium is intimately involved in the mechanism of renin secretion.

Stimulatory and inhibitory effects of cyclic AMP on pancreatic glucagon release from monolayer cultures and the controlling role of calcium

When glucagon release from monolayer cultures of newborn rat pancreas was measured over four hours in media containing 2.5 mM Ca++, a significant cyclic AMP-related inhibition of release was observed. This was noted whether intracellular cyclic AMP levels were raised by the addition of exogenous cyclic AMP or dibutyryl cyclic AMP, by phosphodiesterase inhibition with theophylline, or by the stimulation of adenylate cyclase with cholera toxin. The inhibition was concentration dependent for cyclic AMP and could not be reproduced by the addition of AMP, ADP or ATP. Adenosine also inhibited glucagon release while ATP was stimulatory. From time course studies it appeared that the inhibitory effects of cyclic AMP and cholera toxin were progressive after two hours of incubation. With cholera toxin an early stimulation of glucagon release was observed. The effects of cyclic AMP and cholera toxin on arginine-stimulated glucagon release were to stimulate further the glucagon release during the first hour of the incubation. Thus, the effects of raising intracellular cyclic AMP levels were biphasic in that both an early stimulation and a late inhibition of glucagon release were observed. In examining the nature of these responses a remarkable controlling role for Ca++ was uncovered: at Ca concentrations of 0.3 mM and lower no effect of cyclic AMP on glucagon release was found. With 1 mM Ca++ in the medium cyclic AMP stimulated glucagon release early (30 min) and thereafter had no further effect. In the presence of 2.5 mM Ca++ cyclic AMP did not stimulate early but did cause the delayed inhibition of release. It is concluded that the effect of cyclic AMP on glucagon release can be either stimulatory or inhibitory depending upon the Ca++ concentration of the medium and the duration of exposure to raised cyclic AMP levels.