Contribution of Na+ and membrane depolarization to contraction induced by adrenaline in the guinea pig vas deferens

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Abstract
The contribution of Na+ and membrane depolarization to biphasic contractions induced by adrenaline were investigated in the smooth muscle of guinea pig vas deferens. Adrenaline (5 × 10−6 M) produced an initial small contraction (first contraction) followed by a large tonic contraction (second contraction) with subsequent rhythmic activity. The entire response to adrenaline was largely inhibited by phentolamine (5 × 10−6 M). By adding an appropriate concentration of Mn2+ (2 × 10−4 M) or nifedipine (3 × 10−7 M), a Ca2+ blocker, the second contraction was strongly reduced, accompanied by abolishment of the rhythmic contraction, whereas the first contraction was virtually unaffected. However, the first contraction was markedly suppressed by a higher concentration of Mn2+. All contractions produced by adrenaline were greatly reduced in Ca2+-free solution containing 0.5 mM EGTA. By lowering external Na+ concentration, the first contraction was markedly increased without greatly affecting the second contraction. By exposure to Na+-free isotonic high K+ solution, which elicited a greater depolarization of the membrane, the first contraction produced by adrenaline was also greatly potentiated, while the second and rhythmic contractions were eliminated. These results suggest that the adrenaline-evoked first contraction may be due to an influx of membrane bound Ca2+ which is independent of membrane depolarization, while the second (rhythmic) contraction is due to an influx of extracellular Ca2+ which is dependent upon depolarization.