Muscarinic Stimulation Increases Na+Entry in Pancreatic B-Cells by a Mechanism Other than the Emptying of Intracellular Ca2+Pools

G Protein-independent Activation of an Inward Na+Current by Muscarinic Receptors in Mouse Pancreatic β-Cells

“…The amplitude of the Na ϩ current elicited by ACh is small but is sufficient to account for the 15-mmol/liter increase in [Na ϩ ] c occurring in ␤-cells after 15 min of stimulation with 100 mol/ liter ACh (15,25). Indeed, 100 mol/liter ACh activated a mean inward current of 0.77 pA/pF, which corresponds to 6.15 pA for an average ␤-cell capacitance of 7.9 Ϯ 0.08 pF (estimated from 644 ␤-cells tested in the present study).…”

“…Because the inward current activated by ACh is a specific Na ϩ current, we termed it I Na-ACh . Mechanisms of Activation of I Na-ACh -As in other cells (37,38), lowering the Ca 2ϩ content of the endoplasmic reticulum in ␤-cells activates conductances for Ca 2ϩ and perhaps other ionic species including Na ϩ (25,39). Even if such a mechanism slightly contributes to the current, it is not responsible for I Na-ACh , because ACh still activated the inward current after emptying of the endoplasmic reticulum Ca 2ϩ stores by thapsigargin.…”

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).…”

“…In this regard, it is pertinent that Na + can permeate the store-operated channels (Worley et al, 1994a;Worley et al, 1994b). Increased Na + entry actually explains the depolarizing effect of muscarinic stimulation that triggers voltage-dependent Ca 2+ entry (Gagerman et al, 1980;Henquin et al, 1988;Saha and Hellman, 1991;Gilon and Henquin, 2001), although Na + channels other than store-operated channels probably contribute to this depolarization (Miura et al, 1996;Gilon and Henquin, 2001). …”

Evidence for Contribution by Increased Cytoplasmic Na+ to the Insulinotropic Action of PACAP38 in HIT-T15 Cells

“…To examine this possibility, we measured [Na ϩ ] cyt by using the fluorophore SBFI in HIT-T15 cells. Measurement of [Na ϩ ] cyt in insulin-producing cells by using SBFI has been performed previously by several groups showing that glucose, glyceraldehyde, and acetylcholine increase the [Na ϩ ] cyt , which might contribute to the insulinotropic action of these secretagogues (25,26,32,33). In our hands, measurement of [Na ϩ ] cyt in suspensions of HIT-T15 cells by using SBFI did not yield reliable results, despite extensive trials in our laboratory (data not shown).…”