Effect of 4‐aminopyridine on release of noradrenaline from the perfused cat spleen by nerve stimulation.

(1968) showed that high K+ evokes secretion of noradrenaline (NA) from the sympathetic nerves of the cat spleen in a calciumdependent manner. It has also been reported that agents which increase the duration of the action potential, such as tetraethylammonium (TEA) and 4-aminopyridine (4-AP), markedly potentiate the release evoked by nerve stimulation (Thoenen, Haefely & Staehelin, 1967;Gillespie & Tilmisany, 1976;Wakade, 1980) Wakade (1981aWakade ( , 1982 further extended these observations in the guinea-pig heart, showing that TEA (20 mM) enhanced the response to 35 and 70 mM K+ i) The Macmillan Press Ltd 1983 278 S.M. KIRPEKAR, J.C. PRAT & M.T. SCHIAVONE by 5 fold and 1.5 fold, respectively, and that TTX blocked this potentiation. They suggested that a part of the release induced by high K+ is due to regenerative activity, since the response was partially blocked by TTX and potentiated by TEA.The primary purpose of this study was to reassess the secretory response to excess K+ in the cat spleen. Our previous investigations of K+-evoked NA release in the cat spleen focused on responses to very high K+ concentrations (bolus injection of 3.7 M K+ or prolonged application of 140 mM K+) (Kirpekar & Wakade, 1968;Kirpekar et al., 1976; Kirpekar etal., 1977;Garcia, Kirpekar & Pascual, 1978). In the present study, we have used intermediate concentrations of K+ to re-examine the role of regenerative depolarization in K+-evoked secretion and the mechanisms of muscarinic inhibition and aautoinhibition in the splenic nerve terminals. Methods Spleen slicesCats were anaesthetized with ether. The abdomen was opened by a midline incision, then the spleen was quickly removed and cut into transverse sections (0.5 to 0.7 mm thick), using a tissue slicer. To label the NA stores of the splenic nerve, the slices were incubated for 30 min with 10 ml of Krebs solution containing 10 pCi of (±)-[3H]-NA (specific activity 7.5 Ci/mmol), then washed 3 times in 20 ml of Krebs solution over a 30 min period. In each experiment, a number of slices (approximately 100 mg) were incubated in 10 ml of Krebs solution for 5 min to determine the background release, then transferred to 10 ml of a test solution for 5 min. The incubation temperature was 37°C. Perfused adrenal glandLeft and right adrenal glands were prepared for retrograde perfusion at room temperature according to the procedure of Garcia, Hernandez, Horga & Sanchez-Garcia (1980). The perfusion rate was about 1 ml/min. Glands were perfused for about 30 min before the start of each experiment. Perfusion and incubation solutionsKrebs bicarbonate buffer consisted of (mM): NaCl 118, KCl 4.7, CaCl2 2.5, MgSO4-7H2O 1.2, KH2PO4 1.2, NaHCO3 25, and glucose 10. When excess K+ (as K2SO4) was added, an osmotic equivalent of NaCl was concomitantly removed. For studies with La3", 2+or 10mMNa, a Tris-buffered solution was used, consisting of (mM): NaCl 143, KCl 6.7, CaC12 2.5, MgSO4-7H20, 2.4, Tris (hydroxymethyl)-aminomethane (Tris) 5.0, and glucose 10. When NaCl was decreased to 10 mM, 266 mM su...

Section: Introductionmentioning

confidence: 99%

Modification of potassium‐evoked release of noradrenaline by various ions and agents

Kirpekar

Prat

Schiavone

1983

“…They also found that 4-AP was more effective than tetraethylammonium (TEA) in reducing the delayed K-current and that 4-AP could substitute for TEA as a K-conductance blocking agent in the depolarization-release coupling system in the giant synapse. Kirpekar, Kirpekar & Prat (1977) observed the effects of 4-AP on release of noradrenaline from the perfused cat spleen by nerve stimulation and concluded that 4-AP increases noradrenaline output at 5 Hz by about five 0007-1188'80'010099-08 S01.00 fold. They postulated that the mechanism of action of 4-AP is by inactivation of K-current in sympathetic nerves and that it prolongs the duration of the action potential, thereby allowing a greater influx of Ca ion into the neurone to enhance release of noradrenaline.…”

mentioning

confidence: 99%

Actions of 4‐aminopyridine on Vascular Smooth Muscle Tissues of the Guinea‐pig

Hara

Kitamura

Kuriyama

1980

I Effects of 4-aminopyridine (4-AP) and procaine on the membrane and contractile properties of smooth muscle cells of the guinea-pig pulmonary artery and portal vein were observed. 2 The membrane potential and length constant of smooth muscle cells of the guinea-pig pulmonary artery were -53.2 mV and 1.2 mm, respectively, and those of the portal vein were -52.6 mV and 0.71 mm, respectively. The membrane was electrically quiescent in the pulmonary artery and it was electrically active in the portal vein. 3 Both 4-AP and procaine depolarized the membrane, increased the membrane resistance and suppressed the rectifying properties in both tissues. Both agents evoked a graded response from the muscle membranes of the pulmonary artery by outward current pulse. Procaine had a greater effect than 4-AP on the above membrane properties. The results suggest that 4-AP and procaine suppress K-conductance of the muscle membrane, and 4-AP but not procaine increases noradrenaline release from the nerve terminal. Presumably intracellular free Ca concentrations are also modified by these agents. The effects of 4-AP and procaine on the vascular muscle were compared with those on other excitable tissues.

“…A third possibility is that TEA and 4-AP may enhance calcium entry into the sympathetic nerves to reverse guanethidine blockade of NA release. TEA and 4-AP greatly enhance NA release by splenic nerve stimulation (Thoenen et al, 1967;Kirpekar et al, 1972;1976a;1976b), and this appears to be mainly due to the ability of TEA and 4-AP to inactivate potassium current and prolong the duration of the action potential, thereby allowing a greater influx of calcium ions into the neurone. Katz & Miledi (1969) …”

Section: Resultsmentioning

confidence: 99%

“…An altemative suggestion was that guanethidine depresses NA release by limiting the access of calcium to those sites in the postganglionic sympathetic nerve ending with which calcium interacts to cause the release of NA (Kirpekar, Wakade, Dixon & Prat, 1969). The present investigation was undertaken to test this theory and to determine whether tetraethylammonium (TEA) and 4-aminopyridine (4-AP), which enhance NA output, presumably by increasing calcium entry into the sympathetic nerves (Thoenen, Haefely & Staehelin, 1967;Kirpekar, Prat, Puig & Wakade, 1972;Kirpekar, Kirpekar & Prat, 1976a;Kirpekar, Wakade & Prat, 1976b), reverse the neuronal blockade produced by guanethidine. A preliminary account of some of these findings has been published (Kirpekar, Kirpekar & Prat, 1977).…”

Section: Introductionmentioning

confidence: 99%

Reversal of Guanethidine Blockade of Sympathetic Nerve Terminals by Tetraethylammonium and 4‐aminopyridine

Kirpekar

Prat

1978

1 The effect of tetraethylammonium (TEA) and 4-aminopyridine (4-AP) on the inhibitory effect of guanethidine on noradrenaline (NA) release was investigated in the perfused spleen of the cat. 2 Guanethidine blocked the release of NA evoked by nerve stimulation. TEA and 4-AP readily reversed this inhibitory effect, and the NA output was nearly doubled after repeated stimulation of the nerves. On subsequent perfusion with Krebs solution without TEA or 4-AP, the inhibitory effect of guanethidine reappeared. 3 The reversal of guanethidine blockade of sympathetic nerves by TEA and 4-AP is discussed.