Some Characterization of the Responses to Substance P and Other Tachykinins in Rabbit Iris Sphincter Muscle

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Abstract
Contractile respones to substance P, physalaemin and eledoisin, 3 members of the tachykinin family, were compared and characterized in rabbit iris sphincter smooth muscle. Eledoisin and physalemin were .apprx. 5 times more potent than substance P, and the maximum responses to substance P and physalemin were about 85% of those to eledoisin and carbachol. The contractile responses to the 3 tachykinins were not affected by tetrodoxin (3 .times. 10-6 M) and atropine (10-6M). The contractions induced by substance P and physalaemin were well sustained even faster they were thoroughly washed out, whereas the eledoisin-induced contraction was rapidly ceased by removing the agonist from the bathing medium. The sustained contraction evoked by substance P or physalaemin was strongly dependent on extracellular Ca ions. Phenoxybenzamine (2 .times. 10-5 M, 10- min) selectively attenuated the response to eledoisin, but not substance P or physalaemin, and concomitant incubation with excess eledoisin (10-7 M) significantly prevented the inhibitory effect of phenoxybenzamine. The difference between responses to eledoisin and to the other peptides, substance P and physalaemin, may suggest the existence of 2 different receptor subtypes for tachykinins in rabbit iris sphincter smooth muscle.