Are dihydropyridine receptors downregulated in the ischemic myocardium?

We investigated the effect of ischemia on cardiac dihydropyridine receptors, which correspond to L-type sarcolemmal calcium channels.

Isolated working rat hearts were perfused aerobically for 10 min, and then subjected to 10-60 min of global ischemia. Control hearts were perfused aerobically for 30 min. [3H]PN 200-110 binding was measured in the unfractionated homogenate, in a crude membrane preparation and in a microsomal fraction.

In the homogenate obtained from control hearts, the Kd and Bmax averaged 0.23 +/- 0.05 nM and 84 +/- 4 fmol/mg protein, respectively, and ischemia did not produce any significant change in these variables. Similar results were obtained in the crude membrane preparation (Kd = 0.29 +/- 0.08 nM, Bmax = 113 +/- 7 fmol/mg, yield of binding sites = 98 +/- 6%, no significant change in these variables during ischemia). On the contrary, in the microsomal fraction, the Bmax for [3H]PN 200-110 decreased after ischemia (115 +/- 15 fmol/mg after 20 min of ischemia vs. 190 +/- 34 fmol/mg in the control condition, P < 0.05), without any change in the Kd. 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 [3H]quinuclidinyl benzylate and [3H]ouabain binding sites was not reduced in the microsomal fraction obtained ischemic hearts.

The total number of cardiac dihydropyridine binding sites was not downregulated during ischemia, although their distribution after tissue fractionation was slightly modified, possibly reflecting receptor redistribution between different subcellular pools.