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

1 The alpha(1)-adrenoceptor agonist methoxamine acted independently of rece ptor activation to reduce I-to and the sustained outward current in rat ven tricular myocytes, and hKv 1.5 and Ky 4.2 cloned K+ channel currents. Two h undred mu M methoxamine reduced I-to by 36% in the presence of 2 mu M prazo sin, and by 37 and 38% after preincubation of myocytes with either N-ethylm aleimide or phenoxybenzamine (n=6). The EC50 values at + 60 mV for direct r eduction of I-to, hKv 1.5, and Ky 4.2 by methoxamine were 239, 276, and 363 mu M, respectively, with Hill coefficients of 0.87-1.5. 2 Methoxamine accelerated I-to and Ky 4.2 current inactivation in a concent ration- and voltage-dependent manner. Apparent rate constants for methoxami ne binding and unbinding gave K-d values in agreement with EC50 values meas ured from dose-response relations. The voltage-dependence of block supporte d charged methoxamine binding to a putative intracellular site that sensed similar to 20% of the transmembrane electrical field. 3 In the presence of methoxamine, deactivating Ky 4.2 tail currents display ed 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 mu M methoxamine. For this channel, therefore, both open and closed cha nnel block appeared to be important mechanisms for the action of methoxamin e.