Role of Ins(1,4,5)P-3, cADP-ribose and nicotinic acid-adenine dinucleotidephosphate in Ca2+ signalling in mouse submandibular acinar cells

cADP-ribose (cADPr) and nicotinic acid-adenine dinucleotide phosphate (NAAD P) are two putative second messengers; they were first shown to stimulate C a2+ mobilization in sea urchin eggs. We have used the patch-clamp whole-cel l technique to determine the role of cADPr and NAADP in relation to that of Ins(1,4,5)P-3 in mouse submandibular acinar cells by measuring agonist-evo ked and second-messenger-evoked changes in Ca2+-dependent K- and Cl- curren ts. Both Ins(1,4,5)P-3 and cADPr were capable of reproducing the full range of responses normally seen in response to stimulation with acetylcholine ( ACh). Low concentrations of agonist (10-20 nM ACh) or second messenger [1-1 0 muM Ins(1,4,5)P-3 or cADPr] elicited a sporadic transient activation of t he Ca2+-dependent currents: mid-range concentrations [50-500 nM ACh, 50 muM Ins(1,4,5)P-3, or 50-100 muM cADPr] elicited high-frequency (approx. 2 Hz) trains of current spikes; and high concentrations [more than 500 nM ACh, m ore than 50 muM Ins(1,4,5)P-3 or more than 100 muM cADPr] gave rise to a su stained current response. The response to ACh was inhibited by antagonists of both the Ins(1,4,5)P-3 receptor [Ins(1,4,5)P3R] and the ryanodine recept or (RyR) but could be completely blocked only by an Ins(1,4,5)P3R antagonis t (heparin). NAADP (50 nM to 100 muM) did not itself activate the Ca2+-depe ndent ion currents, nor did it inhibit the activation of these currents by ACh. These results show that, in these cells, both Ins(1,4,5)P3R and RyR ar e involved in the propagation of the Ca2+ signal stimulated by ACh and that cADPr can function as an endogenous regulator of RyR. Furthermore, althoug h NAADP might have a role in hormone-stimulated secretion in pancreatic aci nar cells, it does not contribute to ACh-evoked secretion in submandibular acinar cells.