FACTORS INVOLVED IN CAPILLARY GROWTH IN THE HEART

Growth of capillaries in the heart occurs under physiological circumst ances during endurance exercise training, exposure to high altitude an d/or cold, and changes in cardiac metabolism or heart rate elicited by modification of thyroid hormone levels. Capillary growth in all these conditions can be linked with increased coronary blood flow, decrease d heart rate, or both. This paper brings evidence that, although incre ased blood flow due to long-term administration of coronary vasodilato rs results in capillary growth, a long-term decrease in heart rate ind uced by electrical bradycardial pacing in rabbits and pigs, or by chro nic administration of a bradycardic drug, alinidine, in rats, stimulat es capillary growth with little or no change in corollary blood flow. Decreased heart rate results in increased capillary wall tension, incr eased end-diastolic volume and increased force of contraction, and thu s stretch of the capillary wall. This could lead to release of various growth factors possibly stored in the capillary basement membrane. Co rrelation was found between capillary density (CD) and the levels of l ow molecular endothelial cell stimulating angiogenic factor (ESAF) bot h in rabbit and pig hearts with CD increased by pacing. There was no r elation between expression of mRNA for basic fibroblast growth factor and CD in sham-operated and paced rabbit hearts. In contrast, mRNA for TGF beta was increased in paced hearts, and the possible role of this factor in the regulation of capillary growth induced by bradycardia i s discussed.