Sequential growth of fetal sheep cardiac myocytes in response to simultaneous arterial and venous hypertension

1 Heart Research Center, 2 Department of Physiology and Pharmacology, 3 School of Medicine (Cardiovascular Medicine), Oregon Health and Science University and 4 Portland Veterans Affairs Medical Center, Portland, Oregon Submitted 11 July 2006 ; accepted in final form 21 September 2006 While the fetal heart grows by myocyte enlargement and proliferation, myocytes lose their capacity for proliferation in the perinatal period after terminal differentiation. The relationship between myocyte enlargement, proliferation, and terminal differentiation has not been studied under conditions of combined arterial and venous hypertension, as occurs in some clinical conditions. We hypothesize that fetal arterial and venous hypertension initially leads to cardiomyocyte proliferation, followed by myocyte enlargement. Two groups of fetal sheep received intravascular plasma infusions for 4 or 8 days (from 130 days gestation) to increase vascular pressures. Fetal hearts were arrested in diastole and dissociated. Myocyte size, terminal differentiation (%binucleation), and cell cycle activity (Ki-67[+] cells as a % of mononucleated myocytes) were measured. We found that chronic plasma infusion greatly increased venous and arterial pressures. Heart (but not body) weights were 30% greater in hypertensive fetuses than controls. The incidence of cell cycle activity doubled in hypertensive fetuses compared with controls. After 4 days of hypertension, myocytes were ( 11%) longer, but only after 8 days were they wider ( 12%). After 8 days, %binucleation was 50% greater in hypertensive fetuses. We observed two phases of cardiomyocyte growth and maturation in response to fetal arterial and venous hypertension. In the early phase, the incidence of cell cycle activity increased and myocytes elongated. In the later phase, the incidence of cell cycle activity remained elevated, %binucleation increased, and cross sections were greater. This study highlights unique fetal adaptations of the myocardium and the importance of experimental duration when interpreting fetal cardiac growth data. hypertrophy; hyperplasia; terminal differentiation; cardiomyocyte Address for reprint requests and other correspondence: S. Jonker, 1270 CBRB, 285 Newton Rd., Univ. of Iowa, Iowa City, IA 52242-1101 (e-mail: sonnet-jonker{at}uiowa.edu )