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Prolonged Responses by the Rat Uterus in Vivo to Deamino-Oxytocin Disproportionate to Its Estimated Plasma Half-Life^*

DIANA GAZIS, J. ROY and IRVING L. SCHWARTZ

Department of Physiology and Biophysics, Mt. Sinai School of Medicine New York, New York 10029

Abstract

Deamino-oxytocin, oxytocin, and arginine-vasotocin were studied with respect to their actions on milk ejection, blood pressure, and uterine contractions in rats in vivo. In the in vivo uterus preparation, deamino-oxytocin had an extremely protracted action; the increment in response length as the injected dose was doubled (response-length increment) was 26 ± 4 min compared to 3.5 ± 0.4 min for similar doses of oxytocin.

By comparing response-length increments for either pressor and milk ejection or pressor and uterine responses to injections of arginine vasotocin in single rats, we verified that the responselength increments for pressor, uterine, and milk ejection responses can, under some conditions, provide an estimate of plasma half-life.

Response-length increments were then measured for oxytocin and deamino-oxytocin for pressor, uterine, and milk ejection responses. Response-length increments for deamino-oxytocin were short (like those of oxytocin, which had response-length increments similar to its plasma half-life) in all responses, except uterine contractions.

The uterine response to deamino-oxytocin could be inhibited by injections of the specific inhibitor, [l-N^a-acetyl,2-O-methyltyrosine] oxytocin, at all times during the course of the response. When this inhibitor was injected 30 sec before the injection of deamino-oxytocin or oxytocin, inhibitions of the responses to these two peptides were similar in degree.

We conclude from these data that 1) deamino-oxytocin has a short half-life, similar to that of oxytocin; 2) the protracted uterine action of deamino-oxytocin is due to specifically bound peptide which remains in the uterus long after unbound peptide has been cleared from the circulation; and 3) the affinity of deamino-oxytocin for the uterine receptor is not markedly different from that of oxytocin, which does not elicit prolonged uterine responses. (Endocrinology 106: 805,1980)

Footnotes

^* This work was supported by Research Grants AM-10080 and AM-09140 from the NIAMDD and a fellowship from the New York Heart Association.

To whom requests for reprints should be addressed.

Received April 23, 1979.