[Chemistry and biology of hypothalamic cardioactive proteins and peptides]

The data presented concern the chemistry and biology of cardiotrop peptides and proteins isolated by us from the hypothalamus. The molecular mechanisms of the effect of neurohormone "C" (NC) as well as of a new cardiotrop hexapeptide from cattle hypothalamus are discussed. In in vitro studies on homogenates NC has been found to inhibit greatly not only 3'--5'-cyclo-AMP phosphodiesterase activity of brain and heart but also 3'--5'-cyclo-GMP phosphodiesterase activity. NC has been shown to be bound to specific proteins and to the regulatory unit of cyclo-AMP-dependent histone kinase of brain. It seems to compete with cyclo-AMP for the same proteins and is considered to be a regulator of intracellular cyclic nucleotides. NC has been shown to be combined to specific proteins in brain with non covalent bonds. A new cardiotrop hexapeptide has been shown to be present in bovine hypothalamus and its chemical structure has been found to be Tyr-Leu-Gly-Arg-Pro-Gly-amide. The acetylated form of this hexapeptide, which may be also present in brain, is much more active. The radioimmunochemical experiments carried out with antiserum 744 (from prof. Schally) by us have confirmed the existence of this hexapeptide and other fragments of LH-RH in the bovine hypothalamus. The effect of this hexapeptide on cardiac function and metabolism has been compared with a number of polypeptides (luliberin fragments). The hexapeptide has been shown to have not only cardiotropic but also a hypoglycaemic effect. It enhances the secretion of insulin and counteracts the inhibitory action of somatostatin on the insular apparatus. The hexapeptide produces significant changes in the activities of phosphorylase a and b as well as in that of phosphoprotein phosphatases. It reduces the amount of kinines in blood. Certain fractions of substance P, have been shown to have cardiotrop actitivty--they increase the rate of blood leaving the heart. The organotrop effects of a number of peptide neurohormones are discussed in connection with the hexapeptide. The results obtained have shown that the mechanisms underlying the effects of the cardioactive substances found by us are quite different. The data presented show that in brain a number of chemical factors (mainly peptides) are formed, which are involved in the regulation of heart function.