Brane Potential Research Articles

Calcium oscillations in non-excitable cells

but rather repetitive rises in cytosolic free Ca 2+ ([Ca2+]i), stimulated by vasopressin in hepatocytes (Fig. 1A) or histamine in endothelial cells (Fig. 1B), or from a recently published model of receptor- mediated [Ca.,+]i oscillations (Fig. 1C). Not only has [Ca2+]i replaced mV on the vertical axis, but the time axis is in minutes rather than tens of milliseconds; and these are signals generated in non-excitable ceils, i.e. cells that do not have action potentials. These patterns mirror the conversion of an ampli- tude-modulated stimulus into a digital code, so well-known to neuroscientists, for instance with generator potentials producing trains of spikes at sensory nerve endings or constant depolarization evoking repetitive spikes in a motor neurone. There is even a suggestion of a 'pace-maker' rise in [Ca2+] i prior to the rapid upstroke in Fig. 1B and C. The original description of the all-or-nothing law came from E. D. Adrian's work on signal transmission along axons where the frequency and pattern of essentially invariant action poten- tials gives faithful coding over long distances. But we also know that the amplitude and shape of an action potential can be important in the transduction of information, for example when presynaptic inhi- bition reduces spike height and thereby reduces transmitter re- lease, or when the changing dur- ation of a cardiac action potential changes the strength and duration of a systolic contraction. Similar mechanisms are now being un- covered as oscillating [Ca2+]i sig- nals are investigated in non-excit- able cells. It is of course expected that action potentials will evoke spikes of [Ca2+]i in cells with significant voltage-gated Ca .,+ entry, or inter- nal release controlled by mem- brane potential as in skeletal mus- cle. Such spikingin [Ca2+]i has been indirectly monitored in neurones by measuring Ca2+-activated K + conductance, or in muscle by measuring contraction, and more recently by direct measurements of [Ca2+]i with indicator dyes or photoproteins. However, it is only

Effect of mercuric chloride on contractility and transmembrane potential of the guinea-pig myocardium.

1. The effect of HgCl2 in concentrations from 1 times 10(-4) to 4 times 10(-3) M on the force of contraction of the guinea-pig papillary muscles was investigated. A rapidly developing negative inotropic effect was found to be interrupted by a transient increase in contractile force. 2. In papillary muscles from reserpine-pretreated animals only the decrease of force of contraction was observed. Thus the positive inotropic effect is due to an indirect sympathomimetic action of HgCl2. 3. Mercuric chloride in concentrations above 1 times 10(-3) M raised the threshold of stimulation in more than half the muscles. 4. HgCl2 shortened the time to peak force (t1) and the relaxation time (t2); in muscles of animals pretreated with reserpine, however, t2 increased above the control value. 5. When exposed to HgCl2 all muscles developed a contracture beginning during the 5th min. 6. The time course of membrane resting potential, overshoot, and duration of action potential gradually decreased under the influence of mercuric chloride. The direct negative inotropic effect had a slightly faster onset than had the changes of the electrical parameters.

Dual Mechanisms of Salt Uptake in Relation to Compartmentation and Long-Distance Transport

While investigations of salt transport continue to resemble the parable of the blind men and the elephant, many more hands have been laid on of late-in an increasingly sensitive manner-and in consequence a more cohe­ sive and rigorous view o f at l east the phenomenology o f salt transport in higher plants and algae is gradually emerging. The subject in general has been meticulously reviewed at frequent intervals (11, 15, 51, 56) . The r e­ cency of the latest published rev-iews and proceedings of symposia (67, 87) assures that few revelations are to be expected in these pages. Rather it will be my intention to examine a body of material in a personal, and henee a polarized, way, with the hope that such a presentation will elicit the appear­ ance of alternative views, and will help to crystallize, and perhaps in time resolve, many unsettled aspects of the problem. Thus the treatment will be more heuristic than complete, and many noteworthy contributions will be neglected. Where my views are contrary to those o f others, and part icularly where I invoke interpretations of their work not shared by the authors themselves, I would hope that our differences will lead to an ultimate under­ standing which will prove generally satisfying. It was intended originally to present a broad review encompassing sev­ eral areas in the field of salt uptake which have received major attention in the last years. However, to do so would unavoidably have led to superficial coverage, and the choice was made in favor of a more extensive treatment of a limited area, with the expectation that other aspects of the problem will be treated in depth in the future by other authors. Consequently, little will be said directly about recent excellent work pertaining to cell electropo­ tentials and ion transport in higher plant cells ( 16, 26, 34, 42, 62) , nor about investigations dealing wi th compartmentation, specific ion fluxes, and mem­ brane potentials in algal cells ( for example, 15, 18, 28-30, 63, 64, 84, 85) and higher plant cells ( 13, 32, 60, 72, 96) . Provocative work on the coupling of photo syn thesis to ion transport both in algae (64, 85, 86) and in higher plants (43, 44) will unhappily be slighted, as will the gravid relationship between ion uptake and organic acid synthesis (33, 70). Further, attention