Abstract
1 The effects of several vasoactive substances have been studied on blood flow and acetylochline-induced contraction of the vascularly isolated gastrocnemius of the cat. All substances, including acetylcholine, were administered intra-arterially to the muscle. Blood flow and contractile tension were monitored simultaneously.2 All substances which increased blood flow, enhanced the contractile responses to acetylcholine; angiotensin, which decreased blood flow, attenuated them.3 Histamine was typical of the vasodilators in doses up to 1 mug (9 nmol), but in a dose tenfold higher it produced only a small and transient increase in blood flow and little or no enhancement of the acetylcholine-induced contractions. These paradoxical effects were brought about by that portion of the high dose of histamine which re-entered the cat's systemic circulation.4 The order of potency of the vasodilators was the same for their potentiating effect on the acetylcholine-induced contractile responses as for their enhancing effect on muscle blood flow: bradykinin >histamine>>papaverine>>NaH(2)PO(4) >KCl. Median effective doses (ED(50)S) determined only for bradykinin, histamine and papaverine were, respectively, 20.5, 49.8, and 3,352 pmol as potentiators, and 13.4, 199.8 and 85,000 pmol as vasodilators.5 In general, the effect of a given vasoactive substance on the contractile response to acetylcholine was dose-dependent and correlated well with its effect on muscle blood flow at the moment the acetylcholine was injected. Two important exceptions were, firstly, that the highest dose of papaverine was only moderately effective as a potentiator even though highly effective as an increaser of muscle blood flow; and, secondly, that histamine produced its greatest potentiating effect 10 s after it was injected, at which time its effect on muscle blood flow was quite small.6 It is suggested that these exceptions and, indeed, the much greater potentiating effectiveness of histamine and bradykinin versus papaverine may be due to the ability of histamine and bradykinin to increase the permeability of the muscle's capillaries as well as increasing blood flow through them, thus facilitating much better than mere vasodilators the access of bloodborne acetylcholine to its receptors on the muscle fibres.