The intracellular sodium concentration and activity and the extracellular space in isolated frog sartorii

Beta-adrenergic effects on cellular Na, Mg, Ca, K and Cl in vascular smooth muscle: electron probe analysis of rabbit pulmonary artery

The effects of beta-adrenergic stimulation on the cellular content and subcellular distribution of Na, Mg, Ca, K and Cl were determined by electron probe X-ray microanalysis of muscles stimulated with 5-hydroxytryptamine. Isoproterenol caused a significant decrease in cytoplasmic and mitochondrial Na and Cl, and an increase in cytoplasmic Mg. Isoproterenol also significantly decreased total cytoplasmic Ca measured with small diameter probes, without affecting cellular Ca measured with large probes that included the sarcoplasmic reticulum (SR). The decrease in cytoplasmic Na and the effects on cytoplasmic and cellular Ca are consistent with, respectively, beta-adrenergic stimulation of the Na-pump and of Ca-uptake into the SR, but the beta-adrenergic increase in cytoplasmic Mg also raises the possibility of stimulated Na/Mg exchange.

A macro cell-attached patch-clamp study of the properties of the Na current in the vicinity of the motor endplate region of frog single interosseal skeletal muscle fibres

A macro cell-attached patch-clamp technique using large (16 microns) electrodes was developed to study the properties of the Na current (INa) in the motor endplate region of frog interosseal muscle fibres (resting potential approx.-99 mV). The fibre isolation procedure allows the formation of gigaOhm seals and thus a good voltage-clamp control of the patch. At 22 degrees C, in the presence of 110 mM external [Na] and of K channel blockers, INa activates at -49.6 +/- 1.9 mV, reaches a maximum of 4.33 +/- 0.66 mA/cm2 at -7.5 +/- 2.5 mV and reverses at Vrev equal to +63.2 +/- 1.9 mV. There is no evidence for the presence of a tubular INa. When the [Na] in the recording pipette is changed, Vrev exactly follows the Nernst equation for Na ions. An internal [Na] of 9.69 mM is determined. The Na conductance is maximum (63.56 +/- 7.03 mS/cm2) at +8.9 +/- 3.0 mV and markedly decreases for potentials positive to +25 mV. In contrast, the Na permeability (maximum of 6.13 +/- 0.95 x 10(-4) cm/s at +51.4 +/- 5.6 mV) remains more constant at positive potentials. The Na channels are half-inactivated at -68.9 mV and half-activated at -37.9 mV. At the potential at which INa is maximum, the half-time of activation is 223.9 +/- 10.5 microseconds, the time to peak 365.7 +/- 13.5 microseconds and the time constant of inactivation 260.7 +/- 11.2 microseconds. The time constant of reactivation at -100 mV is 1.44 +/- 0.19 ms. These and other results show that INa can be adequately studied with this technique.

Intracellular free magnesium in frog skeletal muscle studied with a new type of magnesium-selective microelectrode: interactions between magnesium and sodium in the regulation of [Mg]i

The application of a new type of intracellular magnesium-selective microelectrode based on the neutral carrier ETH5214 to measure intracellular free magnesium ([Mg]i) in frog skeletal muscle fibers is reported. At room temperature (18-20 degrees C) the average values for [Mg]i was 0.93 mmol/l (pMgi = 3.03 +/- 0.42, SD; n = 38 experiments). The regulation of [Mg]i was studied by measuring [Mg]i and [Na]i with ion-selective microelectrodes during alterations of the membrane potential and the transmembrane sodium and magnesium gradients. Depolarization by increasing external [K] from 2.5 mmol/l to 12.5 mmol/l did not significantly influence [Mg]i. Increasing extracellular [Mg] from 1 mmol/l to 10 and 20 mmol/l caused a concentration-dependent rise in [Mg]i and a decrease in [Na]i, whereas removal of external magnesium did not affect [Mg]i. Removal of external [Na] caused an increase in [Mg]i and a decrease of [Na]i. The results show that [Mg]i in frog skeletal muscle is not in thermodynamic equilibrium and suggest that a Na/Mg exchange mechanism may be involved in maintaining low levels of [Mg]i.