The mechanisms underlying the inotropic effect of reductions in [K+]o were studied using recordings of membrane potential, membrane current, cell shortening and [Ca2+]i in single, isolated cardiac myocytes. followed by a slower, gradual reduction in the amplitude of cell shortening. After about 2 min in 1 mm[K+]o, peak shortening had decreased to about 55% of the control value. The unfavorable inotropic effect of [K+]o reduction was reversed by briefly exposing the cell ABT-888 kinase inhibitor to a Tyrode answer where the concentration of extracellular Na+ ([Na+]o) was reduced from 140 to 70 mm by substituting external Na+ with Li+. On return to 5 mm[K+]o, the resting membrane potential immediately depolarized to ?80 mV, but the negative inotropic effect reversed slowly. These results are consistent with previous data reported from our laboratory under voltage-clamp conditions (Bouchard shows an example of the effects of reducing [K+]o from 5 to 1 1 mm around the action potential waveform and accompanying contraction of a rat ventricular myocyte. In addition to the large membrane hyperpolarization (?76 to ?106 mV), the 90% duration of the action potential increased from 89 to 253 ms. Peak shortening was reduced to 33% of its control value. In 14 cells, reduction of [K+]o from 5 to 1 1 mm 1 mm resulted in a change in resting membrane potential from ?79.4 1 mV (mean s.e.m.) to ?112.3 1 mV, an increase in the duration of the action potential at 90% repolarization (APD90) from 51.5 5.4 to 129.3 13 ms, and a decrease in peak single-ended unloaded cell Rabbit polyclonal to ZNF544 shortening to 43.3 4% of control (from 4.2 0.6 m to 1 1.9 0.3 m). The effects of changes in [K+]o on action potentials and cell shortening of current-clamped rabbit ventricular and atrial cells are compared in Fig. 3. Reduction of [K+]o from 5 to 1 1 mm resulted in hyperpolarization of the resting potential of the rabbit ventricular cell by about 35 mV, a shortening of the original stage of repolarization, and an extremely proclaimed prolongation of last repolarization that was frequently accompanied by huge afterdepolarizations (Fig. 3illustrates regular types of the membrane contractions and currents documented from rat ventricular, rabbit rabbit and ventricular atrial myocytes in some keeping potentials. As illustrated with the pooled data in Fig. 4illustrates the result of changing keeping potential on [Ca2+]we in a consultant rat ventricular myocyte. The myocyte was activated to contract utilizing a group of 200 ms depolarizing pulses to +20 mV, which is certainly close to the peak from the currentCvoltage romantic relationship for L-type Ca2+ stations in these cells (Bouchard displays the result of reducing [K+]o from 5 to at least one 1 mm on unloaded cell shortening and membrane currents of the rat ventricular cell that was voltage clamped at ?80 mV, near its resting potential. Reduced amount of [K+]o somewhat elevated the magnitude from the transient outward element of membrane current through the depolarizing stage, and created an outward change in the keeping current. This change in keeping current is certainly in keeping with the difference in currentCvoltage relationships for 0.017). Body 6shows that little positive inotropic impact was in addition to the keeping potential over the number ?60 to ?120 mV. As shown in physique 6and similar results were obtained in rabbit ventricular and atrial myocytes. Reduction of [K+]o from 5 to 1 1 mm resulted in a small positive inotropic effect, an approximately 12% increase in peak cell shortening in ventricular cells and a ABT-888 kinase inhibitor 9% increase in atrial cells. Conversely, an increase in [K+]o (to 10 mm) resulted in a small unfavorable inotropic effect in both cell types (Fig. 6 0.2) at either [K+]o. The increase in cardiac contractility following reductions of [K+]o has been typically attributed to reduced Ca2+ efflux or increased Ca2+ influx by the sarcolemmal Na+CCa2+ exchanger as a result of the increase in intracellular Na+ concentration following inhibition of the Na+CK+-ATPase ABT-888 kinase inhibitor (Eisner shows the effects of membrane hyperpolarization on Ca2+-dependent membrane currents and cell shortening of a voltage-clamped rat ventricular cell. Ca2+-dependent difference currents consisted of a large, rapidly activating inward current that inactivated during the depolarizing step and a small, slowly declining inward current which followed repolarization of the cell to the holding potential. The large initial component results primarily from current through L-type Ca2+ channels, while the slow tail of current is due to electrogenic Na+CCa2+ exchange (Bridge shows that the Ca2+ currents produced by the action.