EVIDENCE FOR 2 MODES OF CA-2+ 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≅10 or 30 μm, 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≤2.5 μm. In addition, the Ca2+ entry response observed at lower concentrations of Cch (2.5 μm) was completely inhibited by membrane depolarization induced with high K+ (>55 mm) or gramicidin D (1 μm), while membrane depolarization had little or no effect on Ca2+ entry induced by 100 μm Cch. Another muscarinic agonist, oxotremorine-M (100 μm; 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 (∼75%) by the inorganic Ca2+ channel blocker La3+ (25-50 μm) suggesting that Oxo-M primarily mobilizes Ca2+ in these cells by increasing Ca2+ entry. Organic Ca2+ channel blockers (verapamil or diltiazem at 10 μm, nifedipine at 1 μm), had no effect on this response. The Oxo-M induced Ca2+ mobilization response, like that observed at lower doses of Cch, was markedly inhibited (∼70-90%) by membrane depolarization (high K+ or gramicidin D). At 100 μm Cch the formation of inositol trisphosphate (IP3) was increased 55% above basal levels. A low concentration of carbachol (1 μm) elicited a smaller change in IP3 formation (∼25%), similar to that seen with 100 μm Oxo-M (∼20%). Taken together, these results suggest that there are two modes of muscarinic receptor-induced Ca2+ entry in HSG-PA cells. One is associated with IP3 formation and intracellular Ca2+ release and is independent of membrane potential; the other is less dependent on IP3 formation and intracellular Ca2+ release and is modulated by membrane potential. This latter pathway may exhibit voltage-dependent gating. © 1990 Springer-Verlag New York Inc.