Beta-adrenergic responsiveness and cardiac autonomic receptors after implantation of the MtTW15 pituitary adenoma in the rat.

The effects of chronic MtTW15 pituitary adenoma implantation on beta-adrenergic responsiveness, cardiac beta-adrenoreceptors, and muscarinic receptors were studied in the rat. Five weeks after s.c. administration of tissue fragments of the MtTW15 adenoma, there was a 51 and 20% increase in the heart weight and body weight, respectively, and a 49-fold increase in the serum prolactin level as compared to the controls. At this time there was also an attenuation in the adenoma-bearing group of the ability of isoproterenol to produce a dipsogenic response and to increase the heart rate. In contrast, isoproterenol stimulated cardiac ornithine decarboxylase (ODC) activity 4.2-fold in both the control and adenoma-bearing groups. There was no change between the two groups in the cardiac ventricular beta-adrenoreceptor or muscarinic receptor concentration as measured by specific (-)-[125I]iodocyanopindolol (CYP) and (-)-[3H]quinuclidinyl benzilate (QNB) respectively. In addition, the concentrations of isoproterenol and carbachol required to inhibit by 50% (IC50) [125I]CYP and [3H]QNB binding, respectively, in the absence of 5'-guanylylimidodiphosphate (Gpp(NH)p) were not different between the two groups. In the presence of Gpp(NH)p, the isoproterenol IC50 value was not different between the two groups, whereas the carbachol IC50 value was increased slightly in the adenoma-bearing group. The data indicate that chronic MtTW15 adenoma implantation attenuated beta-mediated dipsogenic and heart rate responses but had little or no effect on cardiac ODC activity or cardiac autonomic receptor concentrations and agonist binding properties.