Developmental analysis reveals mismatches in the expression of K+ channel α subunits and voltage-gated K+ channel currents in rat ventricular myocytes

Abstract

In the experiments here, the developmental expression of the functional Ca2+-independent, depolarization-activated K+ channel currents, I(to) and I(K), and of the voltage-gated K+ channel (Kv) α subunits, Kv1.2, Kv1.4, Kv1.5, Kv2.1, and Kv4.2 in rat ventricular myocytes were examined quantitatively. Using the whole-cell patch clamp recording method, the properties and the densities of I(to) and I(K) in ventricular myocytes isolated from postnatal day 5 (P5), 10 (P10), 15 (P15), 20 (P20), 25 (P25), 30 (P30), and adult (8-12 wk) rats were characterized and compared. These experiments revealed that mean I(to) densities increase fourfold between birth and P30, whereas I(K) densities vary only slightly. Neither the time- nor the voltage-dependent properties of the currents vary measurably, suggesting that the subunits underlying functional I(to) and I(K) channels are the same throughout postnatal development. In parallel experiments, the developmental expression of each of the voltage-gated K+ channel α subunits, Kv1.2, Kv1.4, Kv1.5, Kv2.1, and Kv4.2, was examined quantitatively at the mRNA and protein levels using subunit-specific probes. RNase protection assays revealed that Kv1.4 message levels are high at birth, increase between P0 and P10, and subsequently decrease to very low levels in adult rat ventricles. The decrease in message is accompanied by a marked reduction in Kv1.4 protein, consistent with our previous suggestion that Kv1.4 does not contribute to the formation of functional K+ channels in adult rat ventricular myocytes. In contrast to Kv1.4, the mRNA levels of Kv1.2, Kv1.5, Kv2.1, and Kv4.2 increase (three- to five-fold) between birth and adult. Western analyses, however, revealed that the expression patterns of these subunit proteins vary in distinct ways: Kv1.2 and Kv4.2, for example, increase between P5 and adult, whereas Kv1.5 remains constant and Kv2.1 decreases. Throughout development, therefore, there is a mismatch between the numbers of Kv α subunits expressed and the functional voltage- gated K+ channel currents distinguished electrophysiologically in rat ventricular myocytes. Alternative experimental approaches will be required to define directly the Kv α subunits that underlie functional voltage-gated K+ channels in these (and other) cells. In addition, the finding that Kv α subunit protein expression levels do not necessarily mirror mRNA levels suggests that caution should be exercised in attempting functional interpretations of observed changes in mRNA levels alone.