Key points are not available for this paper at this time.
In the smooth muscle of the guinea-pig taenia coli the maximum rate of rise of the action potential is low (5-10 v/sec). This has been attributed to a poorly developed sodium carrier system which may be largely inactivated (Holman, 1958; Bulbring & Kuriyama, 1963). The latter is, however, unlikely since Kuriyama & Tomita (1964a & b, 1965) observed that hyperpolarization of the membrane up to 90 mV increased the maximum rate of rise of the spike, but never reached the value observed in skeletal muscle. It is also known that the influence of sodium and calcium ions on spike genera- tion in the smooth muscle membrane is different from that on other excitable cell mem- branes. Bulbring & Kuriyama (1963) found that the rate of rise, but not the amplitude of the spike, was related to the external sodium concentration. On the other hand, both the rate of rise and the amplitude of the spike were a function of the external calcium concentration. Furthermore, in the absence of sodium, the time during which spontaneous discharge continued, as well as the spike amplitude, were a function of the external calcium concentration. These results suggested the possibility that calcium, fixed in the membrane, might not only be important for spike generation by controlling the sodium- carrying channel, but could itself carry the current responsible for the spike.
Kuriyama et al. (Mon,) studied this question.