[Mechanogram profile of the guinea pig intestine. The refractory phase]

In smooth muscle cells, when they are contracted for long time by carbachol, after washing a relaxation occurs. Thereafter if they are stimulated again, a less intense and late contraction develops after a long period of latency. Several drugs used as inducers of the long contraction produce a crossed refractoriness; therefore it would seem that the refractoriness does not depend upon receptors. The block of Na-K pump, elicited by a digitalic agent, potentiates the dose ratio; conversely the latency reaches normal values if a saline solution, with a low NaCl content, is used. On the basis of our results, we hypothesize that during the first phase of the induction very high levels of Na accumulate into the smooth muscle cell; these high Na concentrations, present for few minutes also after the washing, could be responsible for the refractoriness. Since an increase of Ca in the saline solution potentiates the refractoriness, the usefulness of calcium salts, in treating the hyperexcitability of smooth muscle cells could be due to this strengthened effect.