A pivotal role for cADPR-mediated Ca ²⁺ signaling: Regulation of endothelin-induced contraction in peritubular smooth muscle cells

cADPR, a potent calcium-mobilizing intracellular messenger synthesized by ADP-ribosyl cyclases regulates openings of ryanodine receptors (RyR). Here we report that in the rat testis, a functional cADPR Ca ²⁺ release system is essential for the contractile response of peritubular smooth muscle cells (PSMC) to endothelin (ET). We previously showed that this potent smooth muscle agonist elicits intracellular Ca ²⁺ release in PSMC and seminiferous tubule contraction via activation of ETA and ETB receptors. ETB-R induces the mobilization of a thapsigargin-sensitive but IP ₃-independent intracellular Ca ²⁺ pool. Stimulation of permeabilized PSMC with cADPR was found to elicit large Ca ²⁺ releases blocked by either a selective antagonist of cADPR or a RyR blocker, but not by heparin. Western blotting and confocal fluorescence microscopy indicated the specific expression of type 2 RyR in perinuclear localization. ET was found to stimulate the activity of ADP-ribosyl cyclase. Microinjection of the selective cADPR antagonist 8NH ₂-cADPR completely abolished subsequent stimulation of Ca ²⁺ signaling via ETA and ETB receptors. cADPR therefore appears to have an obligatory role for ETA-R and ETB-R-mediated calcium signaling in PSMC. However, ETB-R seem to be coupled exclusively to cADPR whereas ETA-R activation may be linked to IP ₃ and cADPR signaling pathways.