EFFECTS OF K+ AND K+-INDUCED POLARIZATION ON (DV-DT)MAX, THRESHOLD POTENTIAL, AND MEMBRANE INPUT RESISTANCE IN GUINEA-PIG AND CAT VENTRICULAR MYOCARDIUM

We studied the non-membrane potential-dependent effect of K+ on (dV/dt)max and threshold potential in guinea pig and cat ventricular myocardium. Membrane potential (MP) was changed uniformly in segments (length < 1.0 mm) of papillary muscles by applying extracellular polarizing current pulses across a single sucrose gap. Control [K+]. was 5.4 ITLM and test [K+]0 values were 2.0,10.0, 11.5, 13.0, 16.2, 20, 22, and 24.0 nun. Each muscle was studied under four conditions: (1) control [K+]0 and unaltered (control level) resting MP (Em); (2) one of the test [K+]. values and the unaltered (test level) Em; (3) the same test [K+]0 and Em held at the control level; (4) control [K+]« and E» held at the test level. At all [K+]0> 11.5 mix, (dV/dt)max showed a decrease significantly (P< 0.01) greater than the corresponding MP-dependent decrease in both guinea pig and cat myocardium. This non-MP-dependent decrease averaged 7.5% at 11.5 mM, 26.5% at 13.0 mm, 37.2% at 16.2 nw, and 22.7% at 20.0 mm. At [K+]0 > 20.0 mM, (dV/dt)max. was predominantly slow-channel-dependent; it was increased by hyperpolarization to -110 mV at [K+]0= 20 and 22 mM but not at [K+]0= 24mM. Threshold potential became progressively less negative with increasing [K+]0, but this effect was dependent only on MP. The membrane input resistance (rm) was determined by two opposing factors: At a given [K+]0, rm increased with depolarization; and at a given MP, rm - decreased with increasing [K+]0 Our study shows that non- MP-dependent depression of (dV/dt)max in the ventricular myocardium occurs at [K+]0, concentrations that may be encountered in vivo. Ore Res 44:800-814, 1979. Copyright © 1979 American Heart Association, Inc. All rights reserved.