COOLING AS A STIMULUS TO SMOOTH MUSCLES

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Abstract
Rapid or slow cooling acts as a stimulus to contraction for various smooth muscles, including the nictitating membrane of the cat, the ureter and retractor penis muscle of the dog, the intestine and uterus of the pregnant rat, and with cooling below approx. 10[degree]C, the intestine of the frog. When degrees of rapid cooling are plotted against heights of contraction, one obtains an S-shaped "cooling-response" curve, which falls off with extreme degrees of cooling. The muscle "adapts" or "accommodates" to the cooling stimulus, as indicated by partial relaxation at constant temp, after rapid cooling and by a lessened response observed with gradual cooling. Several lines of evidence indicate that, although the mode of action of cooling as a stimulus is unknown, cooling does act on a different excitatory system than epinephrine, or else acts directly on the contractile mechanism, as follows: (a) in the retractor penis muscle, the adrenolytic agent Dibenamine reverses the action of epinephrine, which then tends to produce lengthening, but cooling still elicits a contraction; (b) the separate effects of epinephrine and of cooling are roughly additive when applied simultaneously, but after Dibenamine the effects tend to cancel (algebraic addition); (c) the contraction and respnse curves obtained for the denervated nictitating membrane with cooling differ from those obtained with epinephrine.
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