Dependency of detrusor contractions on calcium sensitization and calcium entry through LOE-908-sensitive channels

1 The subcellular mechanisms regulating stimulus-contraction coupling in detrusor remain to be determined. We used Ca2+-free solutions, Ca2+ channel blockers, cyclopiazonic acid (CPA), and RhoA kinase (ROK) inhibitors to test the hypothesis that Ca2+ influx and Ca2+ sensitization play primary roles. 2 In rabbit detrusor, peak bethanechol (BE)-induced force was inhibited 90% by incubation for 3 min in a Ca2+-free solution. By comparison, a 20 min incubation of rabbit femoral artery in a Ca2+-free solution reduced receptor-induced force by only 5%. 3 In detrusor, inhibition of sarcoplasmic reticular (SR) Ca2+ release by 2APB, or depletion of SR Ca2+ by CPA, inhibited BE-induced force by only 27%. The CPA-insensitive force was abolished by LaCl3. By comparison, 2APB inhibited receptor-induced force in rabbit femoral artery by 71%. 4 In the presence of the non-selective cation channel (NSCC) inhibitor, LOE-908, BE did not produce an increase in [Ca2+](i) but did produce weak increases in myosin phosphorylation and force. 5 Inhibitors of ROK-induced Ca2+ sensitization, HA-1077 and Y-27632, inhibited BE-induced force by similar to 50%, and in combination with LOE-908, nearly abolished force. 6 These data suggest that two principal muscarinic receptor-stimulated detrusor contractile mechanisms include NSCC activation, that elevates [Ca2+](i) and ROK activation, that sensitizes cross bridges to Ca2+.