SINGLE-CELL ENZYME-ACTIVITY AND PROLIFERATION IN THE GROWTH-PLATE - EFFECTS OF GROWTH-HORMONE

Longitudinal growth is a result of proliferation and differentiation o f chondrocytes in the growth plate. Growth hormone (GH) stimulates lon gitudinal growth, and GH receptors have been shown on growth plate cho ndrocytes, but the effects of GH on chondrocytes of different cell lay ers are not char. To study the effect of GH on chondrocyte activity, i n situ biochemical techniques were used to measure enzyme activities, which are associated with cell differentiation (alkaline phosphatase [ ALP]) and osteoclast activity (tartrate-resistant acid phosphatase [TR AP]), within single cells of the growth plate. Uptake of bromodeoxyuri dine (BrdU) was used as a parameter for proliferative activity. In add ition, glucose-6-phosphate dehydrogenase (G6PD) was measured since inc reased proliferation has been associated with increased G6PD activity. The role of GH was studied in a model of isolated GH deficiency (dwar f rat) and complete pituitary deficiency (hypophysectomized rat). Grou ps of GH-deficient dwarf rats were infused with recombinant human GH i n either a continuous or a pulsatile manner, since the pattern of GH s ecretion is an important regulator of growth in the rat. After 7 days, G6PD activity in proliferative chondrocytes and TRAP activity in oste oclasts was increased, while ALP activity in hypertrophic chondrocytes was decreased. GH not only increased the number of chondrocytes that incorporated BrdU but also the total number of chondrocytes in the pro liferative zone; therefore, its ratio, the labeling index (an indicato r of proliferative rate), was not increased. The widths of the prolife rative and hypertrophic zones were increased by both patterns of GH ad ministration. The width of the resting zone was unaffected by continuo us GH but decreased by pulsatile GH. ALP and TRAP activities were, res pectively, higher and lower in hypophysectomized rats compared with th e GH-deficient animals. Hypophysectomized rats had smaller growth plat es than dwarf rats with a disproportionally wide resting zone, which, like BrdU uptake, was not affected by GH. GH treatment resulted in inc reased TRAP and decreased ALP activity. These results indicate that GH stimulates the commitment of chondrocytes within the resting/germinal layer to a proliferative phenotype (as opposed to stimulating the rat e of chondrocyte proliferation) but only in the presence of other pitu itary hormones. Furthermore, this study shows that enzyme activities w ithin single chondrocytes and osteoclasts are GH-sensitive. The extent to which these effects are direct or mediated by systemic or local gr owth factors remains to be clarified.