Mechanisms of neurokinin A- and substance P-induced contractions in rat detrusor smooth muscle in vitro

To investigate the mechanisms of neurokinin A- and substance P-induced contractions of rat urinary bladder smooth muscle, and to compare them with those of the muscarinic agonist carbachol. Rat urinary bladder strips were suspended under 1 g of tension in a physiological buffer at 37 degreesC, gassed with 95% O-2/5% CO2. Mechanical activity was recorded isometrically during exposure to neurokinin A and substance P. Both agents produced concentration-dependent contractions of smooth muscle strips which were unaffected by tetrodotoxin (1 mumol/L), peptidase inhibitors (captopril, thiorphan and bestatin; 1 mumol/L each) or piroxicam (10 mumol/L). The rank order of potency of agonists was neurokinin A > substance P > carbachol. Contractile responses to neurokinin A and substance P, like the contractile responses to carbachol, were abolished in a nominally Ca2+-free medium and significantly reduced by nifedipine (1 mumol/L). SKF-96365 (60 mumol/L), an inhibitor of receptor-mediated Ca2+ entry, abolished the nifedipine-resistant response to substance P and carbachol, and significantly attenuated the response to neurokinin A. Depleting intracellular Ca2+ stores with thapsigargin (1 mumol/L) significantly attenuated neurokinin A-induced contractions but had no effect on substance P- or carbachol- induced contractions. The Rho-kinase inhibitor, Y-27632 (10 mumol/L), significantly reduced both phasic and tonic components of the contractile responses to neurokinin A, substance P and carbachol. The contractile responses induced by tachykinins in rat urinary bladder smooth muscle strips involve a direct action on smooth muscle and are not modulated by peptidases or prostanoids. Neurokinin A and substance P, like carbachol-induced contractions, depend on extracellular Ca2+ influx largely through voltage-operated and partly through receptor-operated Ca2+ channels. Intracellular Ca2+ release contributes to the contractile response to neurokinin A but appears to have no involvement in substance P- and carbachol-induced contractions. Rho-kinase activation contributes to contractions induced by substance P, neurokinin A and carbachol.