Effects of histamine, high potassium and carbachol on42K efflux from longitudinal muscle of guinea-pig intestine

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Abstract
Shortening and rate of loss of 42K were studied in strips of longitudinal muscle taken from guinea-pig ileum. Carbachol, histamine and raising the external K concentration, [K+]0, to 120 mM in the presence of atropine caused equal maximal shortenings of the muscle, but unequal maximal increases in 42K efflux: maximal 42K effluxes obtainable in response to raised [K+]0 and histamine were .apprx. 2/3 and 1/3, respectively, of the maximal efflux in response to carbachol. In the absence of atropine the increase in 42K efflux produced by 120 mM-[K+]0 was .apprx. 50% larger, probably because of the release of release of acetylcholine from nerve endings in the tissue. If inhibitors of histamine metabolism were applied, or a H2-receptor blocker (cimetidine), the maximum 42K efflux produced by histamine was not increased. An analog of histamine reputed to resist metabolic degradation did not produce a larger increase in 42K efflux than histamine. The smaller maximal effect of histamine on 42K efflux than carbachol may be because it can open fewer ion channels in the smooth muscle membrane. The ratio of the concentrations producing 50% maximal shortening and 50% maximal 42K efflux was .apprx. 1:1.3 for raised [K+]0 but .apprx. 1:20 for histamine and carbachol. Depolarization by raising [K+]0 appears to be less effective in causing tension development than similar depolarizations produced by carbachol or histamine. The relative effects of carbachol, histamine and raised [K+]0 were discussed in the light of their similar depolarizing actions. Increases in 42K efflux did not appear to be caused primarily either by contraction or by depolarization of the muscle. Access of the stimulant to cells and receptors other than those which are superficially situated was suggested as being an important factor in deciding the smaller increase in 42K efflux seen with some stimulants. Histamine receptors may be fewer in number than muscarinic receptors and less able in their activated form to open channels through which K+ can escape.