Pharmacology and thermosensitivity of the dartos muscle isolated from rat scrotum

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Abstract
1 The dartos is a thin sheet of smooth muscle closely associated with the skin of the scrotum. Although known to play an important role in scrotal thermoregulation, there has been no detailed study into the pharmacology, or thermosensitivity, of the dartos from any species. Here, we investigate these two parameters in the isolated dartos muscle from rat. 2 Field stimulation of the rat dartos caused contractions that were abolished by tetrodotoxin, phentolamine and guanethidine, but unaffected by atropine or L-N(G)-nitroarginine. Exogenous noradrenaline also produced contractions blocked by both phentolamine and prazosin. In muscles with raised tone and negated sympathetic function, field stimulation failed to elicit relaxation. The dartos muscle did not contract in response to carbachol, nicotine, histamine, 5-hydroxytryptamine (all up to 100 micro M) or substance P (up to 1 micro M). 3 Contractile responses to field stimulation and noradrenaline were much greater at 30 degrees C compared with 40 degrees C; indeed, contractions to 1 micro M noradrenaline at 30 degrees C were relaxed by around 80% on heating to 40 degrees C. Similar heat-induced relaxations were observed during contractions to both U46619 (100 nM) and high K (70 mM). 4 In contrast, contractile responses to the myosin phosphatase inhibitor calyculin-A (1 micro M), either in the presence or absence of external calcium, were resistant to relaxation by heating. In calcium-free medium at 30 degrees C, U46619 continued to produce contractions that were again relaxed by 80% on heating to 40 degrees C. However, in the presence of calyculin-A, this heat-induced relaxation was greatly reduced. 5 Thus, the rat dartos muscle receives a functional sympathetic innervation and contracts to noradrenaline via alpha-adrenoceptors. There is no functional inhibitory innervation. Experiments with calyculin-A suggest that myosin phosphatase is a major contributor to the marked thermosensitivity of the dartos muscle.