Non-specific action of methoxamine on I(to), and the cloned channels hKv 1.5 and Kv 4.2

Abstract

1. The α1-adrenoceptor agonist methoxamine acted independently of receptor activation to reduce I(to) and the sustained outward current in rat ventricular myocytes, and hKv 1.5 and Kv 4.2 cloned K+ channel currents. Two hundred μM methoxamine reduced I(to) by 36% in the presence of 2 μM prazosin, and by 37 and 38% after preincubation of myocytes with either N-ethylmaleimide or phenoxybenzamine (n = 6). The EC50 values at +60 mV for direct reduction of I(to), hKv 1.5, and Kv 4.2 by methoxamine were 239, 276, and 363 μM, respectively, with Hill coefficients of 0.87-1.5. 2. Methoxamine accelerated I(to) and Kv 4.2 current inactivation in a concentration- and voltage-dependent manner. Apparent rate constants for methoxamine binding and unbinding gave K(d) values in agreement with EC50 values measured from dose-response relations. The voltage-dependence of block supported charged methoxamine binding to a putative intracellular site that sensed ~ 20% of the transmembrane electrical field. 3. In the presence of methoxamine, deactivating Kv 4.2 tail currents displayed a distinct rising phase, and were slowed relative to control, such that tail current crossover was observed. These observations support a dominant mechanism of open channel block, although closed channel block could not be ruled out. 4. Single-channel data from hKv 1.5 patches revealed increased closed times with blank sweeps and decreased burst duration in the presence of drug, and a reduction of mean channel open time from 1.8 ms in control to 0.4 ms in 500 μM methoxamine. For this channel, therefore, both open and closed channel block appeared to be important mechanisms for the action of methoxamine.