Regulation of arachidonic acid turnover by 1,25-(OH)₂D₃ and 24,25-(OH)₂D₃ in growth zone and resting zone chondrocyte cultures

Previous studies have shown that phospholipase A₂ activity in rat costochondral chondrocyte cultures is differentially regulated by 1,25-(OH)₂D₃ and 24,25-(OH)₂D₃. 1,25-(OH)₂D₃ stimulates enzyme activity in growth zone chondrocytes but has no effect on the resting zone chondrocyte enzyme activity. 24,25-(OH)₂D₃ inhibits the resting zone enzyme but has no effect on the growth zone chondrocyte phospholipase A₂. This study examined whether the metabolites affect arachidonic acid turnover in their target cell populations. Incorporation and release of [¹⁴C]arachidonate was measured at various times following addition of hormone to the cultures. Acylation and reacylation were measured independently by incubating half of the [¹⁴C]arachidonate-labeled cultures with p-chloromercuribenzoate. The results demonstrated that the distribution of [¹⁴C]arachidonate in membrane phospholipids differed between growth zone and resting zone chondrocytes and between the plasma membranes and matrix vesicles isolated from the growth zone chondrocyte cultures. Plasma membrane phospholipids were more susceptible to the release of [¹⁴C]arachidomic acid by exogenous phospholipases than were matrix vesicles phospholipids. The effect of 1,25-(OH)₂D₃ on growth zone chondrocytes was observed within 5 min. Incorporation was greatest after 60 min; release was greatest after 30 min. 24,25-(OH)₂D₃ stimulated consistently elevated incorporation throughout the incubation period, peaking at 15 min. Peak release was at 60 min. The results confirm that resting zone chondrocytes and growth zone chondrocytes retain a differential phenotype in culture and demonstrate that matrix vesicles are distinct from the plasma membrane in terms of lipid composition and arachidonic acid incorporation. 1,25-(OH)₂D₃ and 24,25-(OH)₂D₃ appear to stimulate arachidonic acid turnover in their target cells by different mechanisms. Changes in fatty acid acylation and reacylation may be one mode of vitamin D-3 action in cartilage.