Effects of elevated prolactin and its normalization on thyroid hormone, cardiac beta-adrenoreceptor number and beta-adrenergic responsiveness

Subcutaneous inoculation of the prolactin secreting MtTW15 adenoma in male Wistar Furth rats for 4 weeks produced a significant increase in serum prolactin and a corresponding decrease in peripheral beta-adrenergic responsiveness. Both the isoproterenol induced drink and heart rate responses used to assess the beta-adrenergic responsiveness were significantly reduced in the hyperprolactinemic rat. Serum T3 and T4 levels were measured as was cardiac beta-adrenergic receptor number to ascertain if an alteration of thyroid hormone and a resultant decrease in beta-adrenergic receptor number was responsible for the attenuated beta-adrenergic responsiveness. Serum T4 was significantly reduced in the hyperprolactinemic group (1.9 +/- 0.3 microgram%) as compared to the control group (6.4 +/- 0.l5 microgram%). However there was no significant difference in serum T3 or in cardiac beta-adrenergic receptor number between the two groups. Removal of the MtTW15 adenoma resulted in a normalization of serum prolactin, T4, and in the responsiveness of the peripheral beta-adrenergic system within 4-6 weeks. These results indicate that the attenuated beta-adrenergic responsiveness associated with hyperprolactinemia is reversible and not dependent on a reduction in beta-adrenergic receptor number.