Evidence that quantal Ca2+ release in HSG cells is not due to 'all-or-none' release from discrete Ca2+ stores with differing sensitivities to IP3.

We demonstrate that the application of the muscarinic agonist carbachol to the human salivary epithelial cell line HSG elicits the now well-known phenomenon of 'quantal' Ca2+ release; namely, that the application of a submaximal concentration of agonist results in the release of only a portion of the agonist-sensitive intracellular Ca2+ pool. One explanation that has been proposed to account for this effect is that there are multiple intracellular Ca2+ stores, each with a different agonist sensitivity, which release Ca2+ in an 'all-or-none' fashion. We test this hypothesis in intact HSG cells with an experimental protocol designed to preferentially load less-agonist-sensitive stores with 40Ca2+ and more-agonist-sensitive stores with 45Ca2+. However, contrary to the expectations of the above explanation, these cells do not preferentially release 45Ca2+ in response to low concentrations of agonist. Thus our data suggest that quantal Ca2+ release must arise from some other property of the stores or their Ca2+ release channels.