DEVELOPMENTAL ANALYSIS REVEALS MISMATCHES IN THE EXPRESSION OF K-SUBUNITS AND VOLTAGE-GATED K+ CHANNEL CURRENTS IN RAT VENTRICULAR MYOCYTES( CHANNEL ALPHA)

In the experiments here, the developmental expression of the functiona l Ca2+-independent, depolarization-activated K+ channel currents, I-to and I-K, and of the voltage-gated K+ channel (Kv) alpha subunits, Kv1 .2, Kv1.4, Kv1.5, Kv2.1, and Kv4.2 in rat ventricular myocytes were ex amined quantitatively. Using the whole-cell patch clamp recording meth od, the properties and the densities of I-to and I-K in ventricular my ocytes 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 increas e fourfold between birth and P30, whereas I-K densities vary only slig htly. Neither the time- nor the voltage-dependent properties of the cu rrents vary measurably, suggesting that the subunits underlying functi onal I-to and I-K channels are the same throughout post-natal developm ent. In parallel experiments, the developmental expression of each of the voltage-gated K+ channel alpha subunits, Kv1.2, Kv1.4, Kv1.5, Kv2. 1, and Kv4.2, was examined quantitatively at the mRNA and protein leve ls using subunit-specific probes. RNase protection assays revealed tha t Kv1.4 message levels are high at birth, increase between P0 and P10, and subsequently decrease to very lo iv levels in adult rat ventricle s. The decrease in message is accompanied by a marked reduction in Kv1 .4 protein, consistent with our previous suggestion that Kv1.4 does no t contribute to the formation of functional K+ channels in adult rat v entricular myocytes. In contrast to Kv1.4, the mRNA levels of Kv1.2, K v1.5, Kv2.1, and Kv4.2 increase (three- to five-fold) between birth an d adult. Western analyses, however, revealed that the expression patte rns of these subunit proteins vary in distinct ways: Kv1.2 and Kv4.2, for example, increase between P5 and adult, whereas Kv1.5 remains cons tant and Kv2.1 decreases. Throughout development, therefore, there is a mismatch between tile numbers of Kv alpha subunits expressed and the functional voltage-gated K+ channel currents distinguished electrophy siologically in rat ventricular myocytes. Alternative experimental app roaches will be required to define directly the Kv alpha subunits that underlie functional voltage-gated K+ channels in these (and other) ce lls. In addition, the finding that Kv alpha subunit protein expression levels do not necessarily mirror mRNA levels suggests that caution sh ould be exercised in attempting functional interpretations of observed changes in mRNA levels alone.