Purinergic modulation of smooth muscle intracellular Ca2+ concentration

Abstract

Basal, steady‐state Ca2+ levels are determined by the balance between Ca2+ release/influx and uptake/extrusion mechanisms. Extrinsic factors can alter this balance to affect smooth muscle excitability. Extracellular ATP acts as an inhibitory neurotransmitter in gastrointestinal and vascular (portal vein) smooth muscle. This study examined the effect of extracellular ATP on the steady‐state intracellular Ca2+ concentration ([Ca2+]c). Extracellular ATP (1 mM; applied via picopritzer) reversibly decreased steady‐state [Ca2+]c (measured using fluo 4). To assess whether ATP functioned by altering influx or extrusion [Ca2+]c and inward currents were measured simultaneously in voltage‐clamped myocytes during a depolarizing pulse. ATP decreased the peak amplitude without altering the voltage‐dependence of the inward current. The P2 purinoceptor inhibitor, suramin, and the P2Y1 specific inhibitor, MRS2179, each prevented ATP inhibition of steady‐state [Ca2+]c and inward current. Interestingly, suramin and MRS2179 each also increased the magnitude of the inward current per se, indicating tonic ATP inhibition of Ca2+ channels by endogenous ATP. ATP did not alter Ca2+ uptake/extrusion mechanisms. ATP appears to influence Ca2+ influx mechanisms in smooth muscle via P2Y receptors to exert a negative regulation of excitability.