Abstract
The influences of extracellular $Ca^{2+}\;and\;K^+$ upon the spike action potentials were studied in isolated uterine strips of rat. Regular, rhythmic uterine contractions were induced by the administration of oxytocin$(0.2{\sim}0.5\;I.U.)$, and recorded with force transducer. Spike action potentials were extracellularly measured by use of suction electrode, and compared with those recorded intracellularly by glass microelectrode. The results obtained were as follows : 1) The frequency and duration of spike bursts, and the number of spikes in a burst could be analyzed by use of both methods. But the absolute values of membrane potential were not measurable with the suction electrode. 2) The duration of contraction$(CD_{90};\;the\;duration\;of\;90%\;relaxation)$ was lengthened from the control 17.0 sec to 20.6 sec, in parallel with the increase of spike number from the control 21 to 26, as the increase in $Ca^{2+}$ concentration from 2 to 4 mM. 3) The amplitude and frequency of contractions were gradually decreased, simultaneously with the decrease in the number of spikes in a burst, when the $Ca^{2+}-antagonist$, verapamil was administered cumulatively. 4) The number of spikes was changed from the control 15 to 7, in cabs of the administration of ver)'low dose of verapamil$(10^{-6}\;g/l)$. 5) Increase in the numbers of spike bursts was well matched to the increase in frequency of contractions when extracellular $K^+$ was increased.