Effect of Verapamil and Sodium Nitroprusside on the Potassium Contracture of Smooth Muscle Strips from the Reticular Groove of Adult Cattle

The effects of verapamil and sodium nitroprusside on smooth muscle strips from the floor of the reticular groove of adult cattle were studied. The mechanical activity of the muscle strips was recorded isometrically. Depolarization was induced with cumulative KCl concentrations (8.8-42.4 mmol/l) in Tyrode solution. This response was extracellular Ca2+ dependent. Verapamil and sodium nitroprusside caused a concentration dependent reduction of the KCl-induced contraction of the reticular groove muscle preparations; their IC50 were 3.9 x 10(-6) mol/l and 1.4 x 10(-4) mol/l, respectively. Pre-incubation with both antagonists (30 min) before KCl addition showed that sodium nitroprusside (10(-6) mol/l) did not affect, but verapamil (10(-6) mol/l) inhibited the KCl response of the muscle preparations. CaCl2 caused a concentration dependent (0.25-7 mmol/l) contraction of the KCl-depolarized muscle strips. Pre-incubation with verapamil (10(-6) mol/l) and sodium nitroprusside (10(-6) mol/l) before CaCl2 addition inhibited the response for about 80% and 20% respectively. These results indicate that action of verapamil may be related to voltage-dependent channel blockade. However, the nitroprusside action could not be completely attributed to these channels.