INCREASED [H-3] NITRENDIPINE BINDING-SITES IN RAT-HEART DURING ADULT MATURATION AND AGING

To study the effects of maturation and aging on calcium channels, we investigated the characteristics of binding of a radioligand, [H-3]nitrendipine, to relatively pure sarcolemmal membranes from 2-, 12- and 24-month-old Sprague-Dawley rat hearts. Specific binding of [H-3]nitrendipine was saturable, and the Scatchard analysis of the binding revealed a single class of binding sites. Binding of [H-3]nitrendipine to the membrane of 12-month-old-rats was 50-75% greater than to the membrane of 2-month-old young adult rats with no further changes in binding during aging from 12 to 24 months. The maximum number of dihydropyridine binding sites (B(max)) was 70% higher in 12- and 24-month-old rat hearts (0.45 and 0.43 pmol/mg protein) than in 2-month-old rats (0.27 pmol/mg protein). The affinity for [H-3]nitrendipine binding, on the other hand, was similar in all three age groups (K(D) values of 0.27, 0.31 and 0.29 nM in 2-, 12- and 24-month-old rats, respectively, at 25-degrees). Membranes of all three age groups showed a similar degree of enrichment in sarcolemmal marker enzymes, indicating that the difference in membrane purity was not a contributing factor to the observed increase in density. Furthermore, increased binding of [H-3]nitrendipine to the membranes of older rat hearts was observed throughout the purification scheme. Since [H-3]nitrendipine binding sites are considered to be specific sites for voltage-gated Ca2+ channels of the sarcolemma, it is concluded that the density of these channels in the myocardium increases during adult maturation and is maintained through senescence.