Figure 1. Spontaneous and PGF-induced electrical and contractile activity in control solution and in the presence of nifedipine in myometrium obtained from guinea-pigs at mid- and late pregnancy.

Spontaneous action potentials (upper traces) were accompanied by large increases in tension (lower traces). PGF induced an initial spike followed by sustained depolarization at both stages of pregnancy. In nifedipine, commenced at the time indicated by the arrow, spontaneous activity was abolished and the membrane depolarized to a greater extent at mid- compared with at late pregnancy. The sustained membrane potential and tension responses to PGF were largely resistant to nifedipine in mid-pregnancy, while they were considerably reduced at term. Portions of the spontaneous action potentials marked * have been shown on an expanded time scale in Fig. 3.