Seasonal and sexual variations in the responsiveness of rabbit hearts to prolactin

Rabbit hearts perfused by the Langendorff technique were studied. The addition of ovine prolactin (NIH-P-S-10) to the perfusate in a concentration of 50 ng/ml produced rapid increases in both the amplitude and rate of contraction in 33 adult male hearts studied in winter. Prepubertal male animals showed no response, and only 1 out of 12 adult females responded. Pretreatment for 10 days with 2.5 mg/day testosterone propionate led to minimal inotropic but not chronotropic responses in 2 out of 4 prepubertal males and 2 out of 4 adult females to prolactin. Clear responses to prolactin were seen in 5 adult males pretreated with reserpine. Propanolol consistently reversed both the inotropic and chronotropic actions of prolactin. The original experiments were performed in January and February. When tested in May, adult males failed to respond to prolactin and this situation, persisted until October when responsiveness again appeared. The same prolactin preparation and procedures were used throughout indicating that the changes must have been due to seasonal variations in the cardiac responsiveness to the hormone.