Endothelin B receptor Ca2+ signaling in shark vascular smooth muscle: participation of inositol trisphosphate and ryanodine receptors.

In mammals, endothelin receptors are sub-classified into ET(A) receptors (ET(A)R), which are purely constrictive in vascular smooth muscle (VSM), and ET(B)R, which may produce constriction in VSM or dilatation by stimulating the production of nitric oxide (NO) from endothelial cells. In contrast, previous studies suggested that shark VSM is stimulated exclusively by ET(B)R. The Ca(2+) signaling pathways utilized by shark VSM in response to stimulation by endothelin-1 (ET-1) have not previously been investigated. We measured cytosolic Ca(2+) concentration ([Ca(2+)](i)) in fura-2-loaded VSM of anterior mesenteric artery of Squalus acanthias and show that the ET(B)R agonists IRL 1620 and sarafotoxin S6c (SRX) increase [Ca(2+)](i) in VSM to the same extent as ET-1 and ET(B)R appears to be the only ETR subtype in sharks. To investigate the participation of the inositol trisphosphate (IP(3)) receptors (IP(3)R), we utilized two inhibitors of the mammalian IP(3)R, TMB-8 and 2-APB. In Ca(2+)-free Ringer, these agents inhibit the response to ET(B)R agonist stimulation by 71%. The ryanodine-sensitive receptor (RyR) may be activated by low concentrations of ryanodine, by abrupt local increases of [Ca(2+)](i), (calcium-induced calcium release) or by cyclic adeninediphosphate ribose (cADPR). We employed three inhibitors of activation of the RyR, Ruthenium Red, 8-Br cADPR and high concentrations of ryanodine; these agents blocked the [Ca(2+)](i) response to ET(B)R agonist stimulation by a mean of 39%. These data show for the first time that in VSM of the shark, ET(B)R activation stimulates both IP(3)R and RyR, and that cADPR is involved in RyR activation.