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

1. The effects of removing the external Ca ions on the spontaneous and evoked activity of the smooth muscle of the guinea-pig taenia coli were investigated with the double sucrose-gap method.2. In Ca-free Locke solution the membrane was depolarized, the membrane resistance became low, the spike amplitude became small and the mechanical response decreased. In most preparations the electrical and mechanical activity was abolished within 10 min, but in some preparations the electrical activity continued for more than 30 min.3. In Ca-free solution containing 0.1 mM-EGTA, the membrane was depolarized and the electrical and mechanical activity was abolished within 5 min in every preparation. When NaCl was replaced with sucrose, the effects of Ca removal on the spike activity and contraction appeared very slowly and the membrane potential and membrane resistance remained unchanged.4. When Ca was replaced with Mg (2 mM) the spike was blocked within 1 min without depolarization or reduction of the membrane resistance. In Na-deficient (sucrose) solution, the presence of Mg accelerated the disappearance of the spike caused by Ca removal.5. In Ca-free solution containing 0.5 mM-Mg, a spike-like activity was observed without accompanying mechanical response. This activity was blocked by increasing the Mg concentration above 2 mM. It was Na-dependent, since it was abolished by removing Na from the external solution, but it was not influenced by tetrodotoxin (2 x 10(-6) g/ml.).6. It was concluded that calcium has at least two functions, one as current carrier for the action potential and another as controller of the Na permeability of the membrane. It was also suggested that the Ca which is bound at the membrane may be utilized as a source of Ca ions to carry the current for the action potential.

The effects of papaverine on the electrical and mechanical activity of the guinea-pig taenia coli

1. Effects of papaverine (5 x 10(-6)-5 x 10(-5)M) were studied on the spontaneous and evoked electrical and mechanical activity of the guinea-pig taenia coli. In normal Locke solution, papaverine slowed and finally stopped the spontaneous spike discharge, usually with a small hyperpolarization of a few mV.2. Papaverine did not change the membrane resistance in normal Locke solution. It abolished the repetitive firing of spikes during the depolarization in response to a current pulse, but a single spike of normal amplitude could always be evoked.3. The mechanical response elicited by a single spike was reduced by papaverine in normal Locke solution. During the maintained depolarization produced by a long current pulse the tonic component of the contraction was more reduced than the phasic component related to the initial spike activity.4. The effects of papaverine were essentially the same as in normal Locke solution when NaCl was replaced with LiCl, or in excess K (24 mM), except that the membrane resistance was increased by papaverine in the presence of excess K.5. Excess Ca (6 mM) antagonized the effect of papaverine on the mechanical response, but not on the electrical response. When the muscle was depolarized in Ca-free solution, papaverine caused repolarization of the membrane and electrical activity reappeared. This effect was similar to that of Mg (4 mM).6. The mechanism by which papaverine suppresses the spontaneous spike activity is discussed and is compared with that of isoprenaline, from which it appears to differ. The observations indicate that papaverine, like Mg, may bind with a site at the membrane with which Ca normally binds, thereby modifying the electrical and mechanical activity controlled by Ca.

Electrical activity of vertebrate photoreceptors

It has been known since the time of Schultze (1866) that in the vertebrate retina there are two types of photoreceptors, rods and cones, and that they serve different visual functions; rods for scotopic vision, and cones for photopic. The terminology originates from the shape of the outer segments in which the photosensitive pigment molecules are contained. The cone outer segments are conic and taper towards the tips, while the rod outer segments are typically cylindrical. Fig. 1 is a schematic diagram from Brown, Gibbons & Wald (1963) of the ultrastructure of the rod and cone outer segments of the mudpuppy, Necturus, as studied by electron microscopy. Both appear to be made up of a pile of transverse paired membranes. In cones these arise by infolding of the plasma membrane, and in rods they have probably arisen in a similar way, but each pair of membranes is sealed around the edge so as to form a closed double-membrane disc (Sjöstrand, 1961). Because of the universal lamellation within the rod and cone outer segments, it looks as if there were no appreciable intracellular space, but yet Toyoda, Nosaki & Tomita (1969), and Toyoda et al. (1970) were successful in intracellular recording from the outer segments of single rods of the nocturnal gecko and frog.

The action potential in the smooth muscle of the guinea pig taenia coli and ureter studied by the double sucrose-gap method

The configuration of the electrotonic potential and the action potential observed by the double sucrose-gap method was similar to that observed with a microelectrode inserted into a cell in the center pool between the gaps. In the taenia and the ureter, the evoked spike was larger in low Na or in Na-free (sucrose substitute) solution than in normal solution. However, the plateau component in the ureter was suppressed in the absence of Na. In Ca-free solution containing Mg (3-5 mM) and Na (137 mM), the membrane potential and membrane resistance were normal, but no spike could be elicited in both the taenia and ureter. Replacement of Ca with Sr did not affect the spike in the taenia, nor the spike component of the ureter but prolonged the plateau component. The prolonged plateau disappeared on removal of Na, while repetitive spikes could still be evoked. It was concluded that the spike activity in the taenia and in the ureter of the guinea pig is due to Ca entry, that the plateau component in the ureter is due to an increase in the Na conductance of the membrane, and that both mechanisms, for the spike and for the plateau, are separately controlled by Ca bound in the membrane.