Adaptation of the poikilothermic heart to chronic catecholamine-induced overload

Abstract

Excessive release or administration of beta-mimetic catecholamines may induce cardiomegaly, necrotic lesions, and accumulation of connective tissue in the heart of adult homeotherms. The aim of our studies was to analyze whether similar changes can also be observed in the heterogenous heart of poikilothemic animals. Their ventricular myocardium is formed by two different layers: the inner spongious musculature supplied by diffusion from ventricular lumen is covered by an outer compact layer with coronary supply. Sensitivity of the poikilothermic hearts (carp, frog, turtle) to necrogenic doses of isoproterenol (IPRO, 2 × 40 mg/kg/48 h) was significandy lower than in homeotherms. Necrotic lesions, if present, were localized in the inner spongious musculature, which exhibits higher activities of enzymes connected with aerobic oxidation. Repeated administration of lower doses of IPRO (5mg/kg/day, for 10 days) did not influence the total weight of the fish heart (Cyprinus carpio) but significandy increased the absolute weight of the outer compact layer and thus also its proportion (from 33% to 43%) in the whole ventricle. In both layers, the changes were accompanied by a higher water content, an increase of isomyosin with a lower ATPase activity, and an increase of collagenous proteins (types I and III). It may be concluded that the response of the poikilothermic heart to catecholamine-induced overload differs significandy from that in the mammalian myocardium: it results in the remodeling of the cardiac structure without development of cardiac hypertrophy. Whether the described increase of the compact/spongious ratio is the first step or the only mechanism of the adaptation of the poikilothermic heart to overload remains a matter of speculation.