Regulation of cytosolic calcium and insulin secretion by galanin and ATP receptors: interactions of pertussis-toxin-sensitive and -insensitive signalling pathways

Abstract

In a previous study it was found that the expression of the exogenous fMet-Leu-Phe-receptor (NFPR) in the insulin-secreting cell line RINm5F mediates inhibition of hormone release and additionally raises cytosolic calcium concentration ([Ca2+]i) by activating phospholipase C (PLC) in a pertussis-toxin (PTX)-sensitive manner. We investigated whether an endogenous receptor could elicit similar effects and examined the interaction with PTX-insensitive signalling pathways. The hormone galanin inhibited insulin release at subnanomolar concentrations and increased [Ca2+]i, mainly by a PTX-sensitive mechanism with an EC50 (50 nM) comparable with that for hyperpolarization of membrane potential. The effect of galanin or fMet-Leu-Phe on [Ca2+]i was inhibited by pre-activation of the P2-receptor by ATP, which mobilizes calcium in a PTX-insensitive fashion. Simultaneous activation of the P2- and peptide receptors caused additive increases in [Ca2+]i saturating at a calcium concentration corresponding to the optimal ATP response. This suggests a specific convergence of PTX-sensitive and -insensitive pathways. In contrast, galanin and FMLP inhibited the insulin secretion induced by ATP (1-100 microM), but only when added prior to the nucleotide. In permeabilized cells, FMLP added after the calcium stimulus still inhibited secretion, indicating that the inefficacy observed in intact cells was not due to the rapid ATP-evoked rise in [Ca2+]i. Thus, (i) insulin-secreting cells possess an endogenous PTX-sensitive pathway mobilizing [Ca2+]i, (ii) inhibitory hormones preferentially activate different effectors depending on the agonist concentration and (iii) activation of NFPR or galanin receptor reveals an unusual dissociation between [Ca2+]i rises and insulin secretion, pointing towards an overriding inhibitory control of exocytosis.

Galanin inhibits glucagon-like peptide-1 secretion through pertussis toxin-sensitive G protein and ATP-dependent potassium channels in rat ileal L-cells

“…Similar results were reported on the secretion of insulin from the tumoral cell line RINm5F (Amiranoff et al 1988, Lang et al 1994 and from rat pancreatic B-cells (Nilsson et al 1989). Our data are also in concordance with those obtained in the rat islet cell Rin-m and in the enteroendocrine STC-1 cell lines.…”

Subunit composition of G(o) proteins functionally coupling galanin receptors to voltage-gated calcium channels.

“…Low specificity of our PCR primers cannot explain these results since the primers we used detected y7 subunit expression in rat brain, Galanin-induced inhibition of Ca2+ channel currents in RINm5F cells was observed previously (Homaidan et al, 1991;Schmidt et al, 1991). The decrease in Ca2+ entry may be one of the mechanisms besides stimulation of ATP-dependent K+ channels (de Weille et al, 1988;Dunne et al, 1989), inhibition of adenylyl cyclase (Amiranoff et al, 1988) and direct inhibition of secretion (Ullrich and Wollheim, 1989) which together lead to inhibition of stimulated insulin secretion in RINm5F cells (Ullrich and Wollheim, 1989;Lang et al, 1994). In contrast, galanin was shown to be a stimulatory hormone on pituitary function, where it increases basaland oestrogen-stimulated prolactin release from lactotrophes (Wynick et al, 1993b); it also stimulated growth hormone secretion after infusion into man and rat (Bauer et al, 1986).…”

“…Bartfai et al (1993b), using another chimeric receptor antagonist, also divided galanin receptors into two different groups, one expressed in the central nervous system, called GL-1, and another one expressed in pancreatic cells, called GL-2. The existence of two galanin receptors is also supported by the findings that an increase in cytoplasmatic calcium is mediated by pertussis toxin (PTX)-sensitive and PTX-insensitive G proteins in rat insulinoma RINm5F cells and small cell lung cancer cells H69 and H510, respectively (Sethi and Rozengurt, 1991;Lang et al, 1994).…”

Effects of physiological quercetin metabolites on interleukin-1β-induced inducible NOS expression

“…Since RINm5F cells do not have type 2 glucose transporter, glucose cannot stimulate insulin secretion [22]. Therefore, ATP was applied to stimulate insulin secretion because RINm5F cells have purinergic P2Y receptor [23]. As shown in Fig.…”