Capacitative calcium entry and TRPC channel proteins are expressed in rat distal pulmonary arterial smooth muscle

Mammalian homologs of transient receptor potential (TRP) genes in Drosophila encode TRPC proteins, which make up cation channels that play several putative roles, including Ca2+ entry triggered by depletion of Ca2+ stores in endoplasmic reticulum (ER). This capacitative calcium entry (CCE) is thought to replenish Ca2+ stores and contribute to signaling in many tissues, including smooth muscle cells from main pulmonary artery (PASMCs); however, the roles of CCE and TRPC proteins in PASMCs from distal pulmonary arteries, which are thought to be the major site of pulmonary vasoreactivity, remain uncertain. As an initial test of the possibility that TRPC channels contribute to CCE and Ca2+ signaling in distal PASMCs, we measured [Ca2+](i) by fura-2 fluorescence in primary cultures of myocytes isolated from rat intrapulmonary arteries (>4th generation). In cells perfused with Ca2+-free media containing cyclopiazonic acid ( 10 muM) and nifedipine ( 5 muM) to deplete ER Ca2+ stores and block voltage-dependent Ca2+ channels, restoration of extracellular Ca2+ (2.5 mM) caused marked increases in [Ca2+](i) whereas MnCl2 (200 muM) quenched fura-2 fluorescence, indicating CCE. SKF-96365, LaCl3, and NiCl2, blocked CCE at concentrations that did not alter Ca2+ responses to 60 mM KCl (IC50 6.3, 40.4, and 191 muM, respectively). RT-PCR and Western blotting performed on RNA and protein isolated from distal intrapulmonary arteries and PASMCs revealed mRNA and protein expression for TRPC1, - 4, and - 6, but not TRPC2, - 3, - 5, or - 7. Our results suggest that CCE through TRPC-encoded Ca2+ channels could contribute to Ca2+ signaling in myocytes from distal intrapulmonary arteries.