A Rise in Intramammary Pressure Follows Electrical Stimulation of Mammary Nerve in Anesthetized Rats*

Abstract

Electrical stimulation of the cut central end of a single mammary nerve in the urethane-anesthetized rat resulted in a rise in pressure in a contralateral mammary gland 8-15 s later. The intramammary pressure (IMP) responses increased in amplitude by increasing the frequency, duration, and voltage of the stimulus; maximal amplitudes were obtained with 1-ms pulses (10-20/s) at 5-30 V applied for 5-10 s, and these could be repeatedly provoked providing 40 s were allowed for the preceding pressure response to subside. All responses were characterized by a single rise and fall in pressure and could be mimicked by a single i.v. injection of 50-400 .mu.U [units] oxytocin. Mammary nerve stimulation in the hydrated lactating rat anesthetized with ethanol also produced a small but repeatable antidiuresis equivalent to that produced by 10-100 .mu.U vasopressin. Injection [i.v.] of 100 .mu.U vasopressin had no effect upon IMP; 500 .mu.U were required to elicit a noticeable rise in pressure. The number of rats responding to electrical stimulation with a rise in IMP increased as the milk volume within the gland increased and also after .beta. -adrenergic blockade with propranolol. No intramammary responses were obtained with mammary nerve stimulation in hypophysectomized mothers or in those rats whose spinal cords were cut above the level of the stimulated nerve, though these rats responded to oxytocin. The rise in IMP pressure in intact rats in response to afferent mammary nerve stimulation is due mainly to oxytocin and, to a lesser degree, to vasopressin released from the neurohypophysis. The importance of local conditions within the gland in determining the force and extent of any contractions produced is also emphasized.