The effect of sodium and calcium on the action potential of the smooth muscle of the guinea‐pig taenia coli

Abstract

SUMMARY1. Spontaneous spike activity and action potentials evoked by external field stimulation were recorded, intracellularly and with the double sucrose gap method, from the smooth muscle of guinea-pig taenia coli.2. Replacement of external NaCl with sucrose (leaving 10 mM-Na in the buffer) caused hyperpolarization and stopped spontaneous activity within 10 min. Spikes could, however, be evoked for 2-3 hr. The amplitude, the overshoot and rate of rise of the spike were increased.3. In 10 mM-[Na]o the intracellular Na concentration was reduced from 35 to 24 mm, shifting the Na-equilibrium potential from + 34 to -22 mV.4. Excess Ca (12-5 mM) caused hyperpolarization and increased membrane conductance. The amplitude and the rate of rise of the spike were increased, the threshold was raised and the latency of the spike evoked by threshold stimulation became shorter.5. The effect of reducing the external Ca concentration depended on the Na concentration present, being greater with higher external [Na]o. When the membrane was depolarized and spikes deteriorated in low Ca (0-2-0*5 mm) reduction of Na to 10 mm caused repolarization and recovery of the action potential.6. Mn (0-5-1*0 mm) blocked spontaneous spike discharge after 20 min. Higher concentrations (more than 2X0 mm) were required to block the evoked action potential.7. The results indicate that the smooth muscle spike in taenia is due to Ca-entry and that Na influences spike activity indirectly by competing with Ca in controlling the membrane potential.

Reactivity to bradykinin and potassium of the isolated duodenum from rats with genetic and renal hypertension

“…The deficiency in Ca2+ handling can be compared to the condition described by Antonio (1968), in which the isolated duodenum equilibrated in low calcium medium presented similar contractile responses. In both conditions the Na' conductance is increased (Brading et al, 1969;Postnov et al, 1979), which favours the contractile component of the response induced by bradykinin (Boschcov et al, 1984).…”

Effects of Ca removal on the smooth muscle of the guinea‐pig taenia coli

“…Na or Mg, compete with Ca for the membrane site, Na or Mg ions in the external solution would be expected to influence the spike or the membrane permeability. It has, in fact, been demonstrated in the taenia (Brading et al 1969b) and in the vas deferens of the guinea-pig (Bennett, 1967) that the spike is larger in a low Na medium than in the normal Na concentration, and that Mg can replace Ca for stabilizing the membrane (Kuriyama & Tomita, 1970).…”

“…The spike in the guinea-pig taenia coli has been shown to be due to Ca entry rather than to Na entry (Brading et al 1969b;Kuriyama & Tomita, 1970). Reduction of the external Na concentration increases the spike amplitude and the rate of rise.…”

“…1). It has been interpreted as being mainly due to an increase in Na conductance (Brading et al 1969b) since removal of Na by replacement with sucrose or Tris caused recovery of the membrane potential and membrane resistance (cf. Fig.…”

Photodynamic effects of erythrosine on the smooth muscle cells of guinea‐pig taenia coli

“…Light intensity and erythrosine concentration are variables which clearly will affect the latency of the response but the delay inherent in the excitation-contraction process itself may also be significant since the photodynamic contraction must involve either a direct activation of the contractile proteins or a time-dependent increase in [Ca]i. In the normal process of contraction an increase in [Ca]i may be accounted for by release from an internal store (Casteels & Raeymaekers, 1979;Den Hertog, 1982), calcium influx due to calcium-dependent action potentials (Imai & Takeda, 1967;Brading et al, 1969;Casteels & Droogmans, 1982) or some other modification of resting membrane permeability (Bolton, 1979).…”