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

1. The blocking action of 4-aminopyridine (4-AP) on a delayed rectifier K,12 K+ channel expressed in oocytes was investigated at room temperature (22°C) and physiological temperature (34°C) using the double-electrode voltage clamp and patch clamp techniques. 2. At room temperature, 4-AP (100 uM) inhibition occurred only after activation of current. The rate of onset of block was dependent upon the length of time current was activated by a depolarizing step. Similarly, removal of block required current activation. The degree of steady-state block by 4-AP was not reduced by increasingly more depolarized step potentials. The degree of steady-state block also did not change over the duration of a I s step.3. When channels were nearly fully inactivated, 4-AP produced no additional block of a subsequent depolarizing step, suggesting that 4-AP did not bind when channels were in the inactivated state. In single channel experiments, 4-AP decreased the mean open time in a dose-dependent manner but did not alter the single-channel current amplitude. 4. At 34°C the I-V relationship and inactivation curve shifted to more negative potentials. Increasing the temperature to 34°C did not alter the degree of block by 4-AP, although the rate of onset of block was greatly enhanced.

Section: Results Suggest That 4-ap Binds To the Open State Of The Kvlmentioning

confidence: 99%

Block by 4‐aminopyridine of a Kv1.2 delayed rectifier K+ current expressed in Xenopus oocytes.

Russell

Publicover

Hart

et al. 1994

“…Based on observations from experiments employing 4-AP to selectively block IK(V), this conductance would appear to be important for repolarization of action potentials, as well as contributing to resting membrane potential in portal vein (Hara et al 1980 J Phy8iol. 495.3 Angiotensin II inhibits smooth muscle 'K(V) 699 (2) Ang is known to activate phosphoinositide-specific phospholipase C leading to the formation of inositol phosphates, DAG and the activation of PKC as assessed by phosphorylation of the 68 and 72 kDa intracellular myristoylated alanine-rich PKC substrate in rat mesenteric arterial smooth muscle cells (Dixon et al 1994); and (3) DAG formation rather than calcium mobilization mediates the enhanced Ang-stimulated vasoconstriction observed in rat aortic rings in the presence of lithium (Ullian, Walsh, Wong & Allan, 1995 (Aiello et al 1996) but PKCC is insensitive to DAG analogues (Ono, Fujii, Ogita, Kikkawa, Igarashi & Nishizuka, 1989) and PdBu (Clement-Chomienne & Walsh, 1996).…”

Section: Electrophysiological Measurementsmentioning

confidence: 99%

Angiotensin II activation of protein kinase C decreases delayed rectifier K+ current in rabbit vascular myocytes.

Clément-Chomienne

Walsh

Cole

1996

1. The effect of angiotensin II (Ang) on delayed rectifier K+ current (IK(v)) was studied in isolated rabbit portal vein smooth muscle cells using standard whole-cell voltage clamp technique. The effect of 100 nM Ang on macroscopic, whole-cell IK(V) was assessed in myocytes dialysed with 10 mm BAPTA, 5 mm ATP and 1 mm GTP either at room temperature or at 30 'C. 2. Application of Ang caused a decline in IK(V) which was reversed upon washout of the drug.Tail current recorded after 250 ms pulses to +30 mV and repolarization to -40 mV was reduced from 3'9 + 0 7 to 2-5 + 0 5 pA pF-' at 20 'C (n = 6) and from 4-5 + 0 5 to 3-13 + 04 pA pF-1 at 30 C (n = 17). 3. Ang had no effect on outward current in the presence of an AT1 selective antagonist, losartan (1 uM), which alone had no direct effect on the amplitude of IK(v). Substitution of extracellular Ca2+ with Mg2+ in the presence of 10 mm intracellular BAPTA did not affect the suppression of IK(V) by Ang. 4. Ang induced a decrease in time constant for the rapid phase of inactivation of the macroscopic current (r1 reduced from 377 + 32 to 245 + 11 ms; T2 unchanged, n = 17).Neither the voltage dependence of activation nor inactivation were affected by Ang. The inhibition of IK(V) by Ang was abolished by intracellular dialysis with the selective PKCinhibitors, calphostin C (1 IBM) and chelerythrine (50 uM). These data provide strong evidence that the decline in IK(V) due to Ang treatment is due to PKC activation. 6. The pattern of expression of PKC isoforms was examined in rabbit portal vein using isoenzyme-specific antibodies: a, e and ; isoenzymes were detected, but /1, y, a and V isoenzymes were not.7. The lack of requirement for Ca2+, as well as the sensitivity of the Ang response to chelerythrine, suggest the involvement of the Ca2P-independent PKC isoenzyme e in the signal transduction pathway responsible for IK(V) inhibition by Ang.Variations in intracellular Ca2+ concentration constitute an important element in the control of vascular smooth muscle tone. Contractile agonists are known to cause an elevation in intracellular Ca2+ concentration by increasing the influx of extracellular Ca2P through voltage-dependent L-type Ca2P channels and/or by releasing Ca2+ from intracellular stores.

“…1980) or rabbit (Leander et al 1977) portal vein, or gastric smooth muscle (exposed to greater than 5 mM-4-AP; Boev et al 1985) is not significantly affected by the o-adrenergic antagonist phentolamine or the neuronal Nat channel blocker TTX. Our results show that the 4-AP-induced oscillations are blocked at concentrations of cholinergic, o-adrenergic, histaminergic and serotonergic antagonists 10-to 100-fold higher than those required to block muscular responses to presynaptic stimulation (Hara et al 1980;Boev et at. 1985).…”

Section: Resultsmentioning

confidence: 71%

“…4-Aminopyridine-induced vasoactivity in the liver may be caused by increased rates of discharge of intrahepatic nerve termini, as has been reported for pulmonary artery (Hara et al 1980) or gastric smooth muscle at a low concentration of 4-AP (0 5 mM; Boev et al 1985). However, 4-AP-induced increases in the contractility of guinea-pig (Hara et at.…”

Section: Resultsmentioning

confidence: 75%

“…Patch-clamp studies of isolated rabbit portal vein smooth muscle cells show four different types of K+ channels, all of which are blocked by extracellular tetraethylammonium (TEA) at concentrations between 0 5 and 20 mm (Hume & LeBlanc, 1989). Isolated rat (Leander, Arner & Johansson, 1977) or guinea-pig (Hara, Kitamura & Kuriyama, 1980) (van Breeman & Saida, 1989 (Adams, Barakeh, Laskey & van Breeman, 1989) (Hill, Pryor, Olson & Dawson, 1987).…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Induction of haemodynamic oscillations in the perfused rat liver by K+ channel blockers.

Hill

Ajikobi

1992

SUMMARY1. Exposure of the isolated perfused (constant flow) rat liver to the K+ channel blockers 4-aminopyridine (4-AP) or Cs+ causes the appearance of oscillations in portal pressure and oxygen uptake. The oscillations have a mean frequency of 0 035 Hz (2-1 cycles/min) and are fully reversible upon perfusion with blocker-free saline. Tetraethylammonium (0-17-24-7 mM) does not induce oscillatory behaviour.2. Reversible block of the 4-AP-induced oscillations is caused by 2 mM-EGTA, or verapamil, chlorpheniramine, phentolamine or propranolol with IC50 values of 0-42, 13-5, 15 or 11-5 /M respectively. The oscillations are transiently blocked by atropine (IC50 = 8-3 /M at peak inhibition) and are not affected by 2-7 tM-tetrodotoxin. 3. Endothelium-dependent vasorelaxants, Kupffer cell activity modifiers, retrograde perfusion, or removal of the portal vein from the circuit do not modify the oscillation parameters.4. Oscillations are also caused by infusion of physiological concentrations of adrenaline or phenylephrine, but not isoprenaline.5. The results provide new evidence for the existence of intrahepatic voltagesensitive Ca2+, and 4-AP-and Cs+-sensitive K+ channels. We propose that the K+ channel blockers reveal an intrinsic oscillator in the liver, and that phasic vasoactivity may involve a minor contribution from neurotransmitter and/or hormonal substances.