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

The subcellular mechanisms regulating stimulus‐contraction coupling in detrusor remain to be determined. We used Ca 2+ ‐free solutions, Ca 2+ channel blockers, cyclopiazonic acid (CPA), and RhoA kinase (ROK) inhibitors to test the hypothesis that Ca 2+ influx and Ca 2+ sensitization play primary roles. In rabbit detrusor, peak bethanechol (BE)‐induced force was inhibited 90% by incubation for 3 min in a Ca 2+ ‐free solution. By comparison, a 20 min incubation of rabbit femoral artery in a Ca 2+ ‐free solution reduced receptor‐induced force by only 5%. In detrusor, inhibition of sarcoplasmic reticular (SR) Ca 2+ release by 2APB, or depletion of SR Ca 2+ by CPA, inhibited BE‐induced force by only 27%. The CPA‐insensitive force was abolished by LaCl 3 . By comparison, 2APB inhibited receptor‐induced force in rabbit femoral artery by 71%. In the presence of the non‐selective cation channel (NSCC) inhibitor, LOE‐908, BE did not produce an increase in [Ca 2+ ] i but did produce weak increases in myosin phosphorylation and force. Inhibitors of ROK‐induced Ca 2+ sensitization, HA‐1077 and Y‐27632, inhibited BE‐induced force by ∼50%, and in combination with LOE‐908, nearly abolished force. These data suggest that two principal muscarinic receptor‐stimulated detrusor contractile mechanisms include NSCC activation, that elevates [Ca 2+ ] i and ROK activation, that sensitizes cross bridges to Ca 2+ .British Journal of Pharmacology (2001) 134 , 78–87; doi: 10.1038/sj.bjp.0704241