Type 1 inositol 1,4,5-trisphosphate receptors mediate UTP-induced cation currents, Ca2+ signals, and vasoconstriction in cerebral arteries.

Zhao G, Adebiyi A, Blaskova E, Xi Q, Jaggar JH

Abstract:

Inositol 1, 4, 5-trisphosphate receptors (IP₃Rs) regulate diverse physiological functions, including contraction and proliferation. Three IP₃R isoforms exist, but functional significance of these subtypes in arterial smooth muscle cells is unclear. Here, we investigated relative expression and physiological functions of IP₃R isoforms in cerebral artery smooth muscle cells. We show that 2-aminoethoxydiphenyl borate and xestospongin C, membrane permeant IP₃R blockers, reduced Ca²⁺ wave activation and global intracellular Ca²⁺ ([Ca²⁺]_i) elevation stimulated by uridine 5-triphosphate (UTP), a phospholipase C-coupled purinergic-receptor agonist. Quantitative PCR, Western blotting, and immunofluorescence indicated that all three IP₃R isoforms were expressed in acutely isolated cerebral artery smooth muscle cells with IP₃R1 being the most abundant isoform at 82 % of total IP₃R message. IP₃R1 knockdown with short hairpin RNA (shRNA) did not alter baseline Ca²⁺ wave frequency and global [Ca²⁺]_i, but abolished UTP-induced Ca²⁺ wave activation and reduced the UTP-induced global [Ca²⁺]_i elevation by ~61 %. Antibodies targeting IP₃R1, and IP₃R1 knockdown, both reduced UTP-induced non-selective cation current (I_cat) activation. IP₃R1 knockdown also reduced UTP-induced vasoconstriction in pressurized arteries with both intact and depleted sarcoplasmic reticulum (SR) Ca²⁺ by ~45 %. These data indicate that IP₃R1 is the predominant IP₃R isoform expressed in rat cerebral artery smooth muscle cells. IP₃R1 stimulation contributes to UTP-induced I_Cat activation, Ca²⁺ wave generation, global [Ca²⁺]_i elevation, and vasoconstriction. In addition, IP₃R1 activation constricts cerebral arteries in the absence of SR Ca²⁺ release by stimulating plasma membrane I_Cat.