CONTRACTION OF THE RABBIT MAMMARY STRIP IN VITRO IN RESPONSE TO OXYTOCIN

Abstract

ABSTRACT The isometric contractile response to oxytocin has been studied in vitro utilizing strips of tissue from rabbit mammary glands. Responses were maximal at resting tensions of 200–400 mg/mm2. The force of contraction increased from the beginning of lactation to about 9 days post partum and then leveled off. Little or no response is noted below 15° C and the maximum response is obtained in the range of 32 to36°C. Irreversible changes begin above 41° C. As little as 0.1 mU oxytocin/ml could be detected and the dose-response curve was demonstrated statistically to be linear between the dosages of 0.5 to 10 mU oxytocin/ml. The doseresponse curve reached a plateau at about 8–11 mU/ml. Replacing sodium with potassium resulted in a logarithmic decrease of contractile force with time. This decrease was partially reversible. The strip became inexcitable in calcium free Tyrode's solution. Excitability was again established by adding calcium. Other than to oxytocin, the mammary strip responded only to acetylcholine. The effect of acetylcholine could be blocked by atropine and this treatment did not affect the response to oxytocin. The site and mechanism of action of oxytocin are discussed.