The stimulatory effects of neurotensin and related peptides in rat stomach strips and guinea-pig atria.

1 The stimulatory effects of neurotensin (NT) and several NT fragments were evaluated in two pharmacological preparations: rat stomach strips and isolated spontaneously beating atria of guinea-pigs.2 In rat stomach strips, NT elicited a dose-dependent contractile effect in concentrations varying between 1.3 x 10(-9) and 5.4 x 10(-7) M.3 The contractile effect of NT (1.3 and 5.4 x 10(-8) M) in this tissue was not modified by atropine (3.4 x 10(-7) M), methysergide (2.0 x 10(-6) M), a mixture of cimetidine (8.0 x 10(-6) M) and diphenhydramine (7.8 x 10(-6) M), indomethacin (1.4 x 10(-5) M), 8-Leu-angiotensin II (1.0 x 10(-6) M), glucagon (2.0 x 10(-6) M) or somatostatin (3.0 x 10(-7) M).4 Rat stomach strips desensitized by bradykinin (6.1 x 10(-6) M) or substance P (7.4 x 10(-6) M) maintained their sensitivities to NT (1.3 and 5.4 x 10(-8) M).5 In guinea-pig atria, NT produced a dose-dependent positive inotropic action in concentrations varying between 5.4 x 10(-10) and 2.7 x 10(-7) M.6 The inotropic effect of NT (2.7 x 10(-9) M) was not influenced by methysergide (2.8 x 10(-6) M), atropine (3.4 x 10(-7) M), practolol (1.5 x 10(-5) M), 8-Leu-angiotensin II (1.0 x 10(-6) M), or indomethacin (1.4 x 10(-5) M), but it was reduced by 37% by cimetidine (4.0 x 10(-5) and 2.0 x 10(-4) M). A combination of cimetidine (4.0 x 10(-5) M) and diphenhydramine (3.9 x 10(-6) M) did not produce a greater inhibition of NT than cimetidine alone.7 Atria desensitized by bradykinin (6.1 x 10(-6) M) or glucagon (2.0 x 10(-6) M) maintained their sensitivities to NT (2.7 x 10(-9) M). Substance P was inactive both as an agonist or antagonist of NT.8 These results suggest the existence of specific NT receptors in rat stomach strips and guinea-pig atria.9 The data derived from our structure-activity study suggest that the minimum structure required for the full stimulation of NT receptors in these two preparations is H-Arg(9)-Pro(10)-Tyr(11)-Ile(12)-Leu(13)-OH. The sequence PyroGlu(1)-Leu(2)-Tyr(3)-Glu(4)-Asn(5)-Lys(6)-Pro(7)-Arg(8)- and the amino acids Ile(12) and Leu(13) appear to contribute mainly to the affinity or binding of NT to its receptor. The chemical groups responsible for the full activation (intrinsic activity) of NT receptors seem to be located in the sequence -Arg(9)-Pro(10)-Tyr(11).