μ-Calpain-mediated deregulation of cardiac, brain, and kidney NCX1 splice variants

μ-Calpain is a Ca2+-activated protease abundant in mammalian tissues. Here, we examined the effects of μ-calpain on three alternatively spliced variants of NCX1 using the giant, excised patch technique. Membrane patches from Xenopus oocytes expressing either heart (NCX1.1), kidney (NCX1.3), or brain (NCX1.4) variants of NCX1 were exposed to μ-calpain and their Na+-dependent (I1) and Ca2+-dependent (I2) regulatory phenotypes were assessed. For these exchangers, I1 inactivation is evident as a Na+i-dependent decay of peak outward currents whereas I2 regulation manifests as outward current activation by micromolar Ca2+i concentrations. Notably, with NCX1.1 and NCX1.4 but not in NCX1.3, higher Ca2+i levels alleviate I1 inactivation. Our results show that (i) μ-calpain selectively ablates Ca2+-dependent (I2) regulation leading to a constitutive activation of exchange current, (ii) μ-calpain has much smaller effects on Na+-dependent (I1) regulation, produced by a slight destabilization of the I1 state, and (iii) Ca2+-dependent regulation (I2) and Ca2+-mediated alleviation of I1 appear to be functionally distinct mechanisms, the latter of which is left largely intact after μ-calpain treatment. The ability of μ-calpain to selectively and constitutively activate Na+-Ca2+ exchange currents may have important pathophysiological implications in tissue where these splice variants are expressed.