A Study on Possible Modulating and Direct Effects of Gamma2-MSH and ACTH-(1-24) on the Cardiovascular System of the Rat

In conscious rats, gamma2-melanocyte-stimulating hormone (gamma2-MSH) dose-dependently increases blood pressure and heart rate, whereas adrenocorticotropin-(1-24) [ACTH-(1-24)] dose-dependently decreases blood pressure, an effect which was accompanied by a reflectory tachycardia. As the exact mechanism involved in these cardiovascular effects of the two melanocortins is as yet not known, we undertook a series of experiments to investigate the possibility that these peptides have modulating or direct effect on the cardiovascular system of the rat. In pithed rats gamma2-MSH, administered intravenously (i.v.) in doses of 5-200 nmol/kg, had no significant effect on systolic and diastolic blood pressure and on heart rate, whereas ACTH-(1-24), 5-500 nmol/kg, i.v., dose-dependently decreased blood pressure and increased heart rate. Infusion of gamma2-MSH, 10(-8) M, or ACTH-(1-24), 10(-6) M, in the isolated perfused rat heart did not significantly affect left ventricular pressure or coronary flow. Pretreatment with either gamma2-MSH or ACTH-(1-24) did not modify the responsiveness of the myocardium and coronary vasculature to salbutamol and phenylephrine. Neither gamma2-MSH nor ACTH-(1-24) did affect the vascular contractile machinery of skinned vascular smooth muscles of the rabbit with respect to Ca2+ handling in the cell, as measured by its sensitivity to exogenously applied Ca2+. Gamma2-MSH had no effect on blood pressure and heart rate in pithed rats in which postganglionic sympathetic outflow was stimulated by 1,1-dimethyl-4-phenylpiperazinium (DMPP), nor in pithed rats in which preganglionic sympathetic outflow was stimulated electrically. A dose of 15 nmol/kg ACTH-(1-24) had no significant influence on preganglionic outflow to the cardiac and vascular structures in pithed rats. These data show that gamma2-MSH does not exert its cardiovascular effects via a peripheral site of action at the level of the vascular system and the heart, nor directly on pre- or postganglionic sympathetic outflow. These results are in support for the notion that the peptide acts via a brain region localised outside the blood-brain barrier. The acute depressor effect of ACTH-(1-24), however, seems to be due to a direct effect on the vasculature in the periphery.