Effects of K+-I and KM nduced Polarization on (dV/dt)max9 Threshold Potential, and Membrane Input Resistance in Guinea Pig and Cat Ventricular Myocardium

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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*]° 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+]o and unaltered (control level) resting MP (E_); (2) one of the test [K+]o values and the unaltered (test level) E.; (3) the same test [K+] and E. held at the control level; (4) control [K+]o and E held at the test level. At all [K+]o 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 mi*. At [K+]o 20.0 mM, (dV/dt)max. was predominantly slow-channel-dependent; it was increased by hyperpolarization to -110 mV at [K+]a 20 and 22 mM but not at [K+]o 24mM. Threshold potential became progressively less negative with increasing [K*]-, but this effect was dependent only on MP. The membrane input resistance (r=) was determined by two opposing factors: at a given [K*]., r_ increased with depolarization; and at a given MP, rdecreased with increasing [K+]o Our study shows that non-MP-dependent depression of (dV/dt)max in the ventricular myocardium occurs at [K+]o, concentrations that may be encountered in vivo. Ore Res 44:800-814, 1979

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