Cessation Of Action Potentials Research Articles

Intracellular calcium transients recorded with Fura-2 in spontaneously active myocytes isolated from the atrioventricular node of the rabbit heart

We have used the fluorescent Ca indicator Fura-2 to assess the changes in intracellular calcium (Cai) in single spontaneously active myocytes isolated from the rabbit atrioventricular node (AVN). Simultaneous recordings of membrane potential and the Fura-2 ratio signal (which reflects Cai) showed that a transient rise of Cai occurred with each spontaneous action potential (AP). The AP upstroke preceded the rise in Cai and repolarization of the AP occurred faster than the decline of Cai. The level of Cai remained raised and progressively declined towards a baseline diastolic level during the subsequent pacemaker depolarization. The Fura-2 (Cai) transient in spontaneously active AVN cells had a time-to-peak of 49.2 +/- 5.4 ms (mean +/- s.e.m.; n = 7) and declined with a single exponential time course (time constant = 139.8 +/- 23.9 ms; n = 7). Application of 10 microM ryanodine completely and irreversibly abolished the Cai transient, identifying the sarcoplasmic reticulum (SR) as the major source of releasable Ca. Both removal of external Ca and block of L-type Ca channels (with 2 microM nifedipine) also abolished Cai transients, suggesting that Ca entry via L-type Ca-channels is involved in triggering the SR Ca release underlying the Cai transient. Removal of external Na (in the presence of 20 microM nifedipine to block L-type Ca channels) caused a reversible increase in Cai, showing that Na/Ca exchange is present in AVN cells and that it is involved in Cai regulation. Spontaneous Cai transients were abolished by 1 microM acetylcholine, and this was associated with a hyperpolarization of membrane potential and cessation of action potentials.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for sodium-calcium exchange in the guinea-pig ureter.

The effects of high-K and low-Na solutions on the smooth muscle of the guinea-pig ureter have been examined in both normal tissues, and tissues in which the Na pump had been blocked by exposure to K-free solutions or ouabain (high-Na tissues). Tension recording, membrane potential measurements and ion analysis were used. High-K solutions depolarize normal tissues, leading to action potential generation and phasic contractions followed, at concentrations greater than 20-30 mM, by cessation of action potentials and the development of a biphasic contracture which declines slowly during continuous exposure. The contracture is abolished by Ca-antagonist drugs, procaine and Ca-free solutions. Short exposures of normal tissues to Na-free solutions do not result in tension development. Longer exposures may initiate tension, depending on the Na substitute used. Sucrose causes depolarization of the cells and spike development associated with phasic contractions, superimposed on a small contracture; Li depolarizes the cells but causes no tension generation; Tris hyperpolarizes the cells and a small increase in basal tone may be seen. On exposure to K-free solutions or ouabain, the tissues do not develop significant tone but their response to short application of high-K solutions grows with time. The tissues also develop the ability to contract on short applications of low-Na solutions. The low-Na contractures are resistant to concentrations of Ca antagonists that abolish the K responses of normal tissues, but are abolished in Ca-free solutions. The ability of the tissues to contract in Na-free solutions is accompanied by an increase in intracellular Na and loss of intracellular K. Even after several hours' exposure to ouabain, however, the tissues still contain significant amounts of K and the membrane potential is the same as, or more negative than that in normal tissues. Therefore it appears that another mechanism, apart from the Na pump, can regulate intracellular Na. On continuous exposure to Na-free solutions, the contracture declines rapidly. The decline is associated with a loss of intracellular Na. The Na-free contracture is larger when K rather than Tris is used as the substitute. This difference persists in the presence of a concentration of Mn that abolishes the K contracture of normal tissues but is abolished by high concentrations (10 mM) of procaine.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of cold storage on the inhibitory action of isoprenaline, phenylephrine and nicotine on the mechanical and membranal activities of guinea-pig taenia caecum.

1. The effects of prolonged cold storage on the mechanical and membranal responses to stimulation of alpha- and beta-adrenoceptors by phenylephrine and isoprenaline, respectively, were studied on the guinea-pig taenia caecum.2. Cold storage invariably caused a decrease in the resting membrane potential, and this effect was enhanced as the duration of treatment was prolonged.3. After cold storage (18 days) the tissue potassium ion content (89.7 +/- 1.7 mmol/kg wet wt.) was decreased to 30.5 +/- 1.9 mmol/kg wet wt. whereas that for sodium (69.2 +/- 1.4 mmol/kg wet wt.) increased to 134.0 +/- 2.3 mmol/kg wet wt.4. In the fresh preparations, phenylephrine (1 and 2 muM) caused a cessation of spontaneous action potentials, accompanied by hyperpolarization of the membrane and relaxation of the muscle. These effects were markedly diminished after 18 days of cold storage. Isoprenaline (1 and 2 muM) also blocked the action potentials and caused a concomitant muscle relaxation, but in most cases the hyperpolarization was not observed. After 14 days of cold storage these mechanical and membranal changes associated with isoprenaline treatment were not demonstrable in most preparations.5. Nicotine (5 muM and 50 muM) produced a biphasic effect, cessation of the action potential, hyperpolarization and subsequent relaxation followed by a long lasting depolarization, an accelerated discharge of action potentials and an increase in muscle tension. After a few days of cold storage the hyperpolarization effect disappeared but the intensity of the long-lasting depolarization as well as the contractile effects were increased. After cold storage for more than 7 days, nicotine did not affect mechanical or electrical activity.6. Dibutyryl 3'5' cyclic AMP (1 muM to 500 muM) failed to affect the mechanical and electrical activities of taenia caecum.7. Phenylephrine and isoprenaline had no effect on the high potassiumdepolarized taenia.8. These observations suggest that the electro-mechanical effect of an alpha-adrenoceptor stimulant on the guinea-pig taenia caecum is more resistant to cold treatment than that of a beta-adrenoceptor stimulant. This inhibitory system of stimulation of both a- and 8-receptors of guinea-pig taenia caecum may react by different mechanisms. The results also demonstrate that cold storage itself changes the membrane permeability to ions and the tissue ion content (Na+ and K+) of smooth muscle of guinea-pig taenia caecum.

Electrical and mechanical activity of the longitudinal muscle of the anterior mesenteric artery of the domestic fowl.

1. The electrical activity and changes in tension of the longitudinal muscle of the anterior mesenteric artery (LMAMA) of the domestic fowl were recorded simultaneously using the sucrose-gap method.2. Spontaneous activity consisted of recurring contractions each accompanied by a burst of action potentials.3. In quiescent preparations, brief electrical stimuli, acetylcholine, or barium chloride produced contractions with the appearance of action potentials. Larger concentrations of barium chloride or acetylcholine produced depolarization and action potentials ceased although contraction was maintained. Whenever depolarization without action potentials occurred, it was associated with a smooth contraction, whereas action potentials were always accompanied by small rapid contractions super-imposed upon the main contraction.4. When the tone was raised with barium chloride (and in the presence of hyoscine) continuous action potentials occurred; under these circumstances brief electrical stimuli or noradrenaline produced relaxation, cessation of action potentials, and hyperpolarization.