ARE DIHYDROPYRIDINE RECEPTORS DOWN-REGULATED IN THE ISCHEMIC MYOCARDIUM

Objective: We investigated the effect of ischemia on cardiac dihydropy ridine receptors, which correspond to L-type sarcolemmal calcium chann els. Methods: Isolated working rat hearts were perfused aerobically fo r 10 min, and then subjected to 10-60 min of global ischemia. Control hearts were perfused aerobically for 30 min. [H-3]PN 200-110 binding w as measured in the unfractionated homogenate, in a crude membrane prep aration and in a microsomal fraction. Results: In the homogenate obtai ned from control hearts, the K-d and B-max averaged 0.23 +/- 0.05 nM a nd 84 +/- 4 fmol/mg protein, respectively, and ischemia did not produc e any significant change in these variables. Similar results were obta ined in the crude membrane preparation (K-d = 0.29 +/- 0.08 nM, B-max = 113 +/- 7 fmol/mg, yield of binding sites = 98 +/- 16%, no significa nt change in these variables during ischemia). On the contrary, in the microsomal fraction, the B-max for [H-3]PN 200-110 decreased after is chemia (115 +/- 15 fmol/mg after 20 min of ischemia vs. 190 +/- 34 fmo l/mg in the control condition, P < 0.05), without any change in the K- d. In this fraction, the yield for PN 200-110 binding sites was 4.7 +/ - 0.6% in the control condition and 2.8 +/- 0.5% after ischemia (P < 0 .05). The yield of other sarcolemmal markers such as [H-3]quinuclidiny l benzylate and [H-3]ouabain binding sites was not reduced in the micr osomal fraction obtained from ischemic hearts. Conclusions: The total number of cardiac dihydropyridine binding sites was not downregulated during ischemia, although their distribution after tissue fractionatio n was slightly modified, possibly reflecting receptor redistribution b etween different subcellular pools.