Electrical properties of the gap junctional membrane studied in rat liver cell pairs.

Cell pairs isolated from adult rat liver were used to study the electrical properties of gap junctions. Each cell of a cell pair was connected to a suction pipette so as to enable whole cell tight-seal recording. A double voltage-clamp approach was adopted to control the voltage gradient across the gap junction and measure the transjunctional current. The current-voltage relationship of the gap junctional membrane was linear over the voltage range tested (+/- 50 m V). Under control conditions, the resistance of the gap junction, rj, was 15 M omega (n = 27; range, 4.6 to 45.8 M omega), corresponding to a conductance gj of 67 nS. rj was insensitive to the nonjunctional membrane potential, Vm (voltage range,-90 m V to + 40 m V). There was no indication of a time-dependent gating of rj (time range, 20 ms to 10 s). Dialysis with 1 mM CaCl2 produced irreversible electrical uncoupling without affecting the linearity of the relationship Vj/Ij.

Sodium/calcium exchange and calcium buffering in mammalian ventricular muscle.

The Na/Ca exchange system was investigated in rapidly perfused small trabeculae (diameter around 250 micron) from ferret ventricle. In these preparations tension and Em were recorded, aiNa, aiK and aiMg were measured using ion-selective microelectrodes. The mean aiNa at room temperature (22-24 degrees C) was 10.9 mmol/l +/- S.D. 4.2 mmol/l. When [Na]o was reduced from 155 mmol/l to 1.5 mmol/l there was a rapid decrease in aiNa (half time 30 s), which was associated with a small increase in resting tension. The decrease in aiNa was not influenced by altering pHo, blocking the Na/K pump and was not passive, for during the decrease the driving force for Na ions was inward. The aiNa decreased if [Ca]o was increased or [Na]o was decreased or vice versa. On the basis of these findings it was concluded that the decrease was due to Na/Ca exchange. Since Ca uptake does occur under Na free perfusion and since [Ca]i hardly changes it was proposed that the mitochondria take up Ca and so prevent an excessive rise in cytoplasmic calcium. The Ca uptake mechanism of the mitochondria is inhibited by [Mg]i, so [Mg]i was measured using ion selective microelectrodes. An essential preliminary step was the determination of aiK since the Mg microelectrodes react not only to Mg but also to Na and K. The mean aiK was 104.4 mmol/l +/- S.D. 9.0 mmol/l. Based on the measured aiNa and aiK, the [Mg]i was 0.4 mmol/l, a level not high enough to markedly inhibit the Ca uptake mechanism of the mitochondria. Criteria for judging the impalement quality when using ion selective microelectrodes are discussed.

Internal potassium activity in ferret ventricular muscle.

Measurements of the K activity (aiK) in ferret ventricle were made using either single- or double-barrelled K-sensitive micro-electrodes. aiK was also estimated from the Nernst equation by measuring the membrane potential when changing the external K concentration. Micro-electrodes filled with the Corning K sensor gave unreliable results most probably due to interference from some substance(s) in the heart cells. Reproduceable measurements were obtained using a valinomycin cocktail as the K sensor. The mean value +/- S.D. for aiK from four experiments which met strict criteria for calibration, electrode penetration and drift was 104 mmol/l +/- 9 mmol/l. If all nine experiments were taken the value was 101 mmol/l +/- 8 mmol/l. Estimation from the Nernst equation gave values of aiK that were on average 20 mmol/l higher than the measured values with the valinomycin cocktail. It is recommended that a valinomycin cocktail be used to measure aiK.