Mobilization of Different Intracellular Calcium Pools after Activation of Muscarinic Receptors in Pancreatic Beta-Cells

Abstract

Exposure to carbachol resulted in a biphasic stimulation of ⁴⁵Ca efflux when β-cell-rich pancreatic islets from ob/ob mice were perifused with a Ca²⁺-deficient medium. The pattern of stimulated 45Ca efflux was markedly modified by glucose. Whereas the initial carbachol-stimulated phase was conditional on previous exposure to glucose, the subsequent phase was completely suppressed by 20 mmol/l of the sugar. The stimulatory phases could be clearly separated also on the basis of a Na⁺ dependence. Removal of extracellular Na⁺ resulted in a disappearance of the second phase, but it was still possible to induce a prominent initial peak if depletion of intracellular K⁺ was prevented when Na⁺ was omitted. It is concluded that activation of muscarinic receptors in the pancreatic β-cells results in mobilization of calcium from more than one intracellular pool. Whereas the second phase of stimulated efflux can be explained in terms of an increased entry of Na⁺ into the β-cells, the initial stimulation may be due to receptor-mediated breakdown of polyphosphoinositides.

Two distinct modes of Ca2+ signalling by ACh in rat pancreatic beta‐cells: concentration, glucose dependence and Ca2+ origin.

“…This is in accord with previous reports that muscarinic agonists induce Ca2+ release from internal stores, transient increases in [Ca2+]i and 45Ca2+ efflux in the absence of extracellular Ca2+ in rat (Yada & Sorimachi, 1989;Theler et al 1992) and mouse fl-cells (Nilsson et al 1987;Gylfe, 1991), mouse islets (Hellman & Gylfe, 1986;Gao et al 1994), and RINm5F cells (Wollheim & Biden, 1986 (Herchuelz & Lebrun, 1994). The inhibition by thapsigargin was specific to the ACh-evoked release of Ca2+, because the [Ca2+]i increase in response to glucose was not affected by the drug (Hamakawa & Yada, 1995 (Thastrup et al 1990;Lytton et at.…”

Oscillatory control of insulin secretion

“…PLC activity was early detected in rodent islets (Schrey and Montague, 1983;Dunlop and Larkins, 1986), and subsequent analyses have demonstrated islet expression of several PLC-β, -γ, and -δ isozymes Zawalich et al, 1995;Gasa et al, 1999;Zawalich and Zawalich, 2000;Kim et al, 2001a;Kim et al, 2001b). The importance of PLC activity for insulin secretion is underlined by the fact that the enzyme is activated not only after exposure of islets and β-cells to various G-protein coupled receptor stimuli, such as acetylcholine/carbachol (Best and Malaisse, 1983;Hellman and Gylfe, 1986a;Best et al, 1987;Biden et al, 1987;Gilon and Henquin, 2001) and ATP (Gylfe and Hellman, 1987;Blachier and Malaisse, 1988), but also after exposure to glucose (Axen et al, 1983;Best and Malaisse, 1983;Laychock, 1983;Montague et al, 1985) and depolarizing agents (Laychock, 1983;Mathias et al, 1985;Best et al, 1987;Biden et al, 1987;Zawalich and Zawalich, 1988 (Prentki et al, 1988;Gylfe, 1991;Hellman et al, 1992;Theler et al, 1992;Miura et al, 1996) and these oscillations are characterized by a much shorter period than the glucose-induced, slow oscillations described above. Most of the [Ca 2+ ] i -elevating effect is due to IP 3 -mediated Ca 2+ mobilization from the ER, but the emptying of the stores also triggers Ca 2+ entry through store-operated channels in the plasma membrane (Liu and Gylfe, 1997;Miura et al, 1997;Dyachok and Gylfe, 2001).…”

Relative Importance of Extracellular and Intracellular Ca2+ for Acetylcholine Stimulation of Insulin Release in Mouse Islets

“…This hydrolysis leads to the formation of inositol 1,4,5-trisphosphate, which mobilizes Ca 2+ from the endoplasmic reticulum (4)(5)(6). Such a mobilization can be detected as a pulse of 45 Ca efflux from islets perifused with a Ca 2+ -free medium (2,(7)(8)(9)(10). It may account for the rise in cytoplasmic free Ca 2+ that 100 |xM carbamylcholine produces in tumoral or normal p-cells in the absence of Ca 2+ or when Ca 2+ channels are blocked by verapamil (3,11).…”