Tension inhibitory effect of lead ions and lead uptake in ileal longitudinal muscle of the guinea-pig.

1. In ileal longitudinal smooth muscle of the guinea-pig, lead ions (Pb2+) inhibited the phasic response to high-K+ (60 mM) solution more strongly than the tonic response. 2. Pb2+ displaced Ca2+ concentration-response curves to the right. Moreover, Pb2+ inhibited the 45Ca binding at high affinity sites determined by the La3+ method than low affinity sites during K(+)-induced contraction. 3. After treatment with Pb2+, the tonic tension caused by high-K+ was fairly restored by a wash with normal medium. The lead bound to the muscle cells was almost eliminated after 20-30 min with a normal medium. 4. We suggest that Pb2+ binds loosely to the surface membrane of ileal longitudinal muscle. Pb2+ probably reduced the tension in response to the high-K+ mainly by inhibiting Ca2+ release rather than by affecting Ca2+ influx.