THE EFFECTS OF BARIUM ION ON THE RESTING AND ACTION POTENTIAL OF INTESTINAL SMOOTH MUSCLE CELL

Mechanism of barium-induced contraction in the vascular smooth muscle of rabbit aorta

In a solution containing 1.5 mM Ca2+, cumulative application of 0.3-10.0 mM Ba2+ induced a concentration-dependent contraction of the rabbit aorta. This contraction was reduced by the Ca2+ channel inhibitors, verapamil (10(-6) M), nifedipine (10(-7) M) and lanthanum (2.0 mM), and was potentiated by the Ca2+ channel facilitator, Bay K8644 (10(-7) M). In a Ca2+-free solution containing EGTA (1.0 mM), cumulative application of Ba2+ still induced a concentration-dependent contraction, the maximum contractile tension of which was comparable to that in the presence of 1.5 mM Ca2+. The Ba2+-induced contraction which was not dependent on the external Ca2+ was also inhibited by verapamil, nifedipine and lanthanum and was potentiated by Bay K8644. A high concentration (65.4 mM) of K+ potentiated this Ba2+-induced contraction whereas noradrenaline (10(-6) M) did not have such an effect. In order to deplete the releasable Ca2+ store in the cell, the muscle strip was treated with noradrenaline (10(-6) M) and/or caffeine (20.0 mM) in a Ca2+-free solution. In such a Ca2+-depleted muscle, Ba2+ still induced a contraction of a similar magnitude to that without such treatment. Further, the second application of Ba2+ in a Ca2+-free solution induced a similar contraction to that induced by the first application of Ba2+. These results suggest that Ba2+ depolarizes the cell membrane and opens the voltage-dependent Ca2+ channels resulting in a Ca2+ influx in the presence of Ca2+. In the absence of external Ca2+, Ba2+ may enter the cell through the voltage-dependent Ca2+ channels and induce contraction without mobilizing the Ca2+ store which is sensitive to noradrenaline and caffeine.

The mechanism of action of Ba2+ and TEA on single Ca2+-activated K+ -channels in arterial and intestinal smooth muscle cell membranes

The interaction of Ba2+ and TEA with Ca2+-activated K+ channels was studied in isolated membrane patches of cells from longitudinal jejunal smooth muscle of rabbit and from guinea-pig small mesenteric artery (100 micron external diameter). Ba2+ applied from the inside of the membrane did not reduce unit current, except at high concentrations, but channels failed to open for long periods (s). This effect became much stronger when the potential gradient was in a direction driving Ba2+ into the channel and was reduced by increasing K+ ion concentration on the outside of the membrane. These results are consistent with Ba2+ entering the open channel and blocking at a site most of the way through the channel bore. In contrast, TEA and procaine dose-dependently reduced unit current amplitude at all patch potentials and slightly increased mean open time. Their effects were not detectably voltage-dependent and could be explained by TEA and procaine blocking the open channel with a timecourse that was faster than the frequency response of the recording system. The lack of appreciable voltage-dependence suggests that TEA and procaine bind to a site near to the inner mouth of the channel.

Barium-induced automaticity in right ventricular muscle in the dog

Pacemaker activity was induced in trabecular muscle fibers of canine hearts using oxygenated Tyrode solution with added barium chloride. Following exposure to barium (0.2 to 5mM) the resting potential decreased toward zero from 10 to 60%, depending on the concentration. Simultaneously, the duration of the action potential increased and early diastolic hyperpolarization appeared with diastolic depolarization characteristic of latent pacemaker activity. In concentrations above 0.75 mM, spontaneous activity appeared with presystolic upsweep at the threshold potential. The changes were reversible when normal Tyrode solution was readministered. Suppression of the induced pacemaker activity occurred on cooling or following imposed rapid stimulation. The changes produced by barium are consistent with a decrease in the conductance of the membrane for potassium.

Nervous factors influencing the membrane activity of intestinal smooth muscle

The effects of various chemical agents on the spontaneous membrane activities and those electrically elicited in the smooth muscles of small intestine were investigated.1. The effects of various chemicals on the spontaneously active membrane might be summarized as follows. (a) Cholinergic agents; atropine slightly hyperpolarized the membrane and reduced the amplitude of slow potential changes even in aged preparations. Prostigmine depolarized the membrane, and enhanced the amplitude and prolonged the duration of the slow potential changes. Atropine prevented the actions of prostigmine on the membrane. (b) Ba(2+) depolarized the membrane, and enhanced the amplitude and prolonged the duration of the slow potential changes. The spike frequency was initially increased, then reduced. Atropine and tetrodotoxin partially prevented the action of Ba(2+) on the membrane activities.2. Effects of chemical agents on the membrane activity elicited by electrical stimulation might be summarized as follows. (a) Short pulse stimulation (0.5-1 msec) generated the spike as a direct response of the muscle cell membrane, then it was followed by slow depolarization, delayed hyperpolarization, i.e. the ;inhibitory potential', and post-inhibitory rebound successively. (b) The slow depolarization and the post-inhibitory rebound were reduced in amplitude by treatment with atropine, and enhanced by treatments with prostigmine and Ba(2+). Tetrodotoxin blocked all activities except the spike.3. When repetitive stimulation (20 c/s) was applied to the membrane, the membrane hyperpolarized; then, after 3-5 sec, it gradually depolarized even if the stimulation was continued, and triggered spikes. The hyperpolarization always preceded depolarization. The duration and the amplitude of the delayed depolarization was proportionally increased by the increased intensity and duration of stimulation. Atropine and tetrodotoxin blocked the generation of the post-inhibitory rebound.4. Effects of repetitive stimulation on the stored tissues were observed. The responses to repetitive stimulation of the membrane of muscles which had been stored 50 hr at 4 degrees C, were the same as those observed in the fresh tissue. The response of the tissue which had been stored 100 hr was the same as that observed in the fresh tissue treated with tetrodotoxin, i.e. all activities except the spikes were blocked.

Effects of barium on mature and embryonic heart cells

Barium chloride was administered to isolated preparations of dog heart and rat atrium and to trypsinized embryonic chick heart cells cultured in vitro. Transmembrane action potentials recorded from pacemaker, specialized conductive and contractile fibers of the dog heart, and atrial fibers of rat heart showed a slowing in the rising and falling phases of the action potential and a prominent negative after-potential. The action potential of the Purkinje fiber of the dog heart showed in addition a depolarization during diastole. A decrease in resting potential was observed later.

The isometric tension developed by the rat atrium showed a significant increase which was greater at the higher concentractions (greater than 3.5 x 10 -4 M) of barium chloride. The cultured embryonic heart cells were appreciably more sensitive to barium than the mature heart cells. An increase in rate of beat of the embryonic heart cells was observed at concentrations of 10 -6 M whereas a change in heart rate was first observed in the adult heart at concentrations above 10 -4 M. The induction of beating in cell communities previously quiescent is due to a local automatic activity rather than to a propagated activity from an adjacent cell community.

The slowing in rise time of the action potential can be attributed to a depression of the sodium-carrying system. The slowing of repolarization can be ascribed to a decrease in potassium conductance. The pronounced negative after-potential can be explained on the basis of sustained impermeability of the membrane to potassium. The mechanism by which barium produces a positive inotropic effect in the rat atrium is open to conjecture. It cannot be attributed solely to a prolongation of the duration of the action potential nor can a direct effect on the contractile proteins be excluded. The presence of extracellular calcium is essential in order to obtain a positive inotropic effect in the rat atrium when barium is added.

Ionic interconversion of pacemaker and nonpacemaker cultured chick heart cells

Trypsin-dispersed cells from hearts (ventricles) of 7 to 8 day chick embryos were cultured 3 to 21 days. The cells became attached to the culture dish and assembled into monolayer communities. By means of a bridge circuit, one microelectrode was used for simultaneously passing current and recording membrane potentials (V(m)). The input resistance, calculated by the measured DeltaV(m) for a known step of current, averaged 10 M ohm. Electrotonic depolarization of nonpacemaker cells had no effect on frequency of firing. Within 2 min after addition of Ba(++) (5 to 10 mM) to the Tyrode bath, the cells became partially depolarized and quiescent nonpacemaker cells developed oscillations in V(m) which led to action potentials. With time, the depolarization became nearly complete and the input resistance increased 2 to 10 times. During such sustained depolarizations, action potentials were no longer produced and often tiny oscillations were observed; however, large action potentials developed during hyperpolarizing pulses. Thus, the automaticity of the depolarized cell became apparent during artificial repolarization. Sr(++) (5 to 10 mM) initially produced hyperpolarization and induced automaticity in quiescent nonpacemaker cells. Elevated [K(+)](o) (20 to 30 mM) suppressed automaticity of pacemaker cells and decreased R(m) concomitantly. Thus, Ba(++) probably converts nonpacemaker cells into pacemaker cells independently of its depolarizing action. Ba(++) may induce automaticity and depolarization by decreasing g(K), and elevated [K(+)](o) may depress automaticity by increasing g(K). The data support the hypothesis that the level of g(K) determines whether a cell shall function as a pacemaker.