Evidence for two modes of Ca2+ entry following muscarinic stimulation of a human salivary epithelial cell line.

We have investigated muscarinic receptor-operated Ca2+ mobilization in a salivary epithelial cell line. HSG-PA, using an experimental approach which allows independent evaluation of intracellular Ca2+ release and extracellular Ca2+ entry. The carbachol (Cch) dose response of intracellular Ca2+ release indicates the involvement of a single, relatively low-affinity, muscarinic receptor site (K0.5 approximately equal to 10 or 30 microM, depending on the method for [Ca2+]i determination). However, similar data for Ca2+ entry indicate the involvement of two Cch sites, one consistent with that associated with Ca2+ release and a second higher affinity site with K0.5 less than or equal to 2.5 microM. In addition, the Ca2+ entry response observed at lower concentrations of Cch (2.5 microM) was completely inhibited by membrane depolarization induced with high K+ (greater than 55 mM) or gramicidin D (1 microM), while membrane depolarization had little or no effect on Ca2+ entry induced by 100 microM Cch. Another muscarinic agonist, oxotremorine-M (100 microM; Oxo-M), like Cch, also induced an increase in the [Ca2+]i of HSG-PA cells (from 72 +/- 2 to 104 +/- 5 nM). This response was profoundly blocked (approximately 75%) by the inorganic Ca2+ channel blocker La3+ (25-50 microM) suggesting that Oxo-M primarily mobilizes Ca2+ in these cells by increasing Ca2+ entry. Organic Ca2+ channel blockers (verapamil or diltiazem at 10 microM, nifedipine at 1 microM), had no effect on this response. The Oxo-M induced Ca2+ mobilization response, like that observed at lower doses of Cch, was markedly inhibited (approximately 70-90%) by membrane depolarization (high K+ or gramicidin D).(ABSTRACT TRUNCATED AT 250 WORDS)