Endothelin B receptor Ca super(2+) signaling in shark vascular smooth muscle: participation of inositol trisphosphate and ryanodine receptors

In mammals, endothelin receptors are sub-classified into ET sub(A) receptors (ET sub(A)R), which are purely constrictive in vascular smooth muscle (VSM), and ET sub(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 sub(B)R. The Ca super(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 super(2+) concentration ([Ca super(2+)] sub(i)) in fura-2-loaded VSM of anterior mesenteric artery of Squalus acanthias and show that the ET sub(B)R agonists IRL 1620 and sarafotoxin S6c (SRX) increase [Ca super(2+)] sub(i) in VSM to the same extent as ET-1 and ET sub(B)R appears to be the only ETR subtype in sharks. To investigate the participation of the inositol trisphosphate (IP sub(3)) receptors (IP sub(3)R), we utilized two inhibitors of the mammalian IP sub(3)R, TMB-8 and 2-APB. In Ca super(2+)-free Ringer, these agents inhibit the response to ET sub(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 super(2+)] sub(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 super(2+)] sub(i) response to ET sub(B)R agonist stimulation by a mean of 39%. These data show for the first time that in VSM of the shark, ET sub(B)R activation stimulates both IP sub(3)R and RyR, and that cADPR is involved in RyR activation.