The electrical and mechanical activities of guinea pig taenia coli smooth muscle were measured by a 'sucrose gap' technique. Under the same experimental conditions the ionic content of smooth muscle was also measured. The mean value of the resting potential was 56.9±1.1 mV (S.E.M.; n=46). In normal Krebs solution immediately after dissection intracellular sodium amounted to 30.1, and intracellular calcium to 1.5 mmolexkg-1 wet weight. In response to adrenaline administration there was a Ca-dependent hyperpolarization (peak, 6.8±0.3 mV S.E.M.; n=5) and an increased Na efflux with a rate constant (k) of 0.16 min-1 (60'). Removal of adrenaline was followed by a so-called 'postadrenaline depolarization', i.e. the decrease of the membrane potential was greater than the initial rise, an effect enhanced by ouabain (2x10-5 M). Clonidine (5.3x10-6 M), a selective presynaptic-adrenoceptor (α2-receptor) stimulant, failed to produce hyperpolarization; however, phenylephrine (5x10-5 M), a pure postsynaptic α-adrenoceptor (α1-receptor) stimulant, produced a similar effect to adrenaline. In addition, yohimbine (1.4x10-6 M), a typical presynaptic α-adrenoceptor inhibitor, failed to affect the action of adrenaline or phenylephrine. These facts indicate that the α1-adrenoceptors present on the smooth muscle are different from those situated presynaptically on the cholinergic nerve terminals modulating the release of acetylcholine. The effect of ouabain to lower membrane potential proved to be Ca2+-dependent. The intracellular sodium content was enhanced by ouabain from 30.1 to 90.9±4.7 mmolexkg-:1 wet weight (60'). On washing out ouabain, 'post-ouabain hperpolarization' was detected, i.e. the rise of the membrane potential was greater than the initial fall. It is suggested that the sodium pump plays a significant role in the post-ouabain hyperpolarization. Direct calculation of sodium movements suggets that the sodium permeability is about 3.7 p-molexcm-2xsec-1 and the pump generates a current of 1.06x10-7 Axcm02 which, in fact, while crossing the smooth muscle membrane with a resistance of 30-60 kΩ cm2 woudl produce a potential difference of 3.2-6.4 mV. It is suggested that the electrogenic sodium pump may contribute to the resting membrane potential about 15 mV, a value obtained from the summation of depolarization induced by ouabain and hyperpolarization produced by adrenaline.
|Number of pages||18|
|Journal||Acta physiologica Academiae Scientiarum Hungaricae|
|Publication status||Published - Dec 1 1980|
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