Recently, we reported indirect evidence that plasma membrane Ca2+-ATPase (PMCA) can mediate B-type Ca2+ channels of cardiac myocytes. In the present study, in order to bring more direct evidence, purified PMCA from human red blood cells (RBC) was reconstituted into giant azolectin liposomes amenable to the patch-clamp technique. Purified RBC PMCA was used because it is available pure in larger quantity than cardiac PMCA. The presence of B-type Ca2+ channels was first investigated in native membranes of human RBC. They were detected and share the characteristics of cardiac myocytes. They spontaneously appeared in scarce short bursts of activity, they were activated by chlorpromazine (CPZ) with an EC50 of 149 mmole/l or 1 mmole/l vanadate, and then switched off by 10 mmole/l eosin or dose-dependently blocked by 1-5 mmole/l ATP. Independent of membrane potential, the channel gating exhibited complex patterns of many conductance levels, with three most often observed conductance levels of 22, 47 and 80 pS. The activation by vanadate suggests that these channels could play a role in the influx of extracellular Ca2+ involved in the vanadate-induced Gardos effect. In PMCA-reconstituted proteoliposomes, nearly half of the ATPase activity was retained and clear "channel-like" openings of Ba2+- or Ca2+-conducting channels were detected. Channel activity could be spontaneously present, lasting the patch lifetime or, when previously quiescent, activity could be induced by application of 50 mmole/l CPZ only in presence of 25 U/ml calmodulin (CaM), or by application of 1 mmole/l vanadate alone. Eosin (10 mmole/l) and ATP (5 mmole/l) significantly reduced spontaneous activity. Channel gating characteristics were similar to those of RBC, with main conductance levels of 21, 40 and 72 pS. The lack of direct activation by CPZ alone might be attributed to a purification-induced modification or absence of unidentified regulatory component(s) of PMCA. Despite a few differences in results between RBC and reincorporated PMCA, most probably attributable to the decrease in ATPase activity following the procedure of reincorporation, the present experimental conditions appear to reveal a channel-mode of the PMCA that shares many similarities with the B-type Ca2+ channel.

The present study demonstrates that B-type Ca2+ channels observed in rat ventricular myocytes markedly reacted to agents known to affect the ion-motive plasma membrane Ca2+-ATPase (PMCA) pump. Chlorpromazine (CPZ)-activated B-type Ca2+ channels were completely blocked by internal application of PMCA pump inhibitors, namely La3+ (100 microm), eosin (10 microm) and AIF(3) (100 microm). Calmodulin (50 U/ml), the main endogenous positive regulator of PMCA, was unable to activate but significantly reduced CPZ-activated B-type channel activity. In the same manner, ATP (1 and 4 mm), the main energizing substrate of PMCA, was able to reversibly and significantly reduce this activity in a dose-dependent manner. Interestingly, anti-PMCA antibody 5F10, but not anti-Na/K ATPase antibody (used as a negative control) induced a marked Ba2+-conducting channel activity that shared the same characteristics with that of CPZ-activated B-type channels. 5F10-Activated channels were mostly selective towards Ba2+ , mainly had three observed conductance levels (23, 47 and 85 pS), were observed with a frequency of about 1 out of 5 membrane patches and were completely blocked by 10 microm eosin. These results suggest that B-type Ca2+ channels are some form of the PMCA pump.

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Rats treated with DOCA salts and subjected to abdominal aortic stenosis display left ventricle hypertrophy associated with a decrease in cardiac I(to) current density and prolongation of the action potential duration. We investigated the molecular basis of these electrophysiological defects by analyzing the amount of mRNA corresponding to the genes encoding the a subunits of the left ventricle K+ channel at the steady state. The mRNAs corresponding to the a subunits of the K+ channel (Kv1.2, Kv1.4, Kv1.5, Kv2. 1, Kv4.2 and Kv4.3) were measured by quantitative RT-PCR using a specific Kv internal standard. In control rats, the Kvl.5 gene was only expressed at a low level, whereas the Kv4.2 and Kv4.3 genes were expressed at a high level. Regardless of the etiology of the hypertrophy, the amounts of Kv1.4 and Kv1.5 mRNAwere similar in treated, sham and control rats. The amounts of Kv1.2 and Kv2.1 mRNA were markedly lower in DOCA-salt treated rats (66%) than in sham-DOCA rats, but no effect was observed after stenosis. The very conservative Kv4.2 and Kv4.3 genes were found to be downregulated simultaneously in both type of hypertrophy. However, the steady-state amount of Kv4 mRNA was even lower in rats with DOCA-salt-induced hypertrophy than in those with stenosis-induced ventricular hypertrophy. Therefore, the decrease in I(to) density, consecutively to pressure- and volume-overload, is due to a large decrease in the amount of Kv4.2 and Kv4.3 mRNA. In addition, DOCA-salt treatment alters the amounts of Kv transcripts independently to cardiac hypertrophy, suggesting that the mineralocorticoid may be involved in Kv gene expression.