Tags

Type your tag names separated by a space and hit enter

Characterization of 4-aminopyridine block of the transient outward K+ current in adult rat ventricular myocytes.
J Pharmacol Exp Ther. 1993 Jun; 265(3):1450-9.JP

Abstract

The blocking action of 4-aminopyridine (4-AP) on the transient outward K+ current (ITO) in isolated rat ventricular myocytes was studied using the whole-cell configuration of the patch-clamp technique. 4-AP inhibition of ITO was concentration-dependent with half-maximal block occurring at 0.2 mM. At high concentrations (> 1 mM), 4-AP appeared to slow both the activation and inactivation phases of ITO. This resulted in a crossover phenomenon where in the presence of 4-AP the outward current was less than control at the beginning of a depolarizing pulse but crossed over during the pulse to become greater than control. Inhibition of ITO by 4-AP was voltage-dependent. Steady-state block of ITO by 4-AP was greatest at or near resting membrane potentials (i.e., -70 mV) but decreased with membrane depolarization. The voltage-dependence of block was steep and was well described by a Boltzmann relationship with a slope factor of approximately 4 mV. The midpoint potential for block was dependent on the concentration of 4-AP, being -41.6 +/- 0.4 mV (n = 9), -40.7 +/- 1.3 mV (n = 6), -34.0 +/- 1.6 mV (n = 5) and -30.1 +/- 0.2 (n = 15) at 0.3, 1, 3 and 10 mM, respectively. The midpoint potential for activation was -12.6 mV and was -46.9 mV for inactivation. The concentration-dependence of the voltage-dependence of 4-AP block can be explained by assuming that the sequential closed states through which the channel passes during activation exhibit successively lower affinities for 4-AP. Onset of ITO block by 4-AP was slow. The association (kON) and dissociation. (kOFF) rate constants for binding at -70 mV were: kON = 207 M-1 s-1 and kOFF = 0.090 s-1. The time constant for unblock (tau UNBLOCK) of ITO at 0 mV was independent of 4-AP concentration indicating that there was no binding of 4-AP at this potential. kOFF (1/tau UNBLOCK) at 0 mV was 2.4 s-1 which is approximately 25-fold faster then at -70 mV. The results suggest that 4-AP binds most strongly to closed channels with the inactivation gate open. The conformational changes that occur during channel opening induce a decrease in affinity for 4-AP so that when the channel is in the open state, 4-AP binding is at its weakest. The processes of 4-AP block and inactivation appear to be mutually exclusive.

Authors+Show Affiliations

Department of Anesthesia Research Laboratories, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts.No affiliation info available

Pub Type(s)

Corrected and Republished Article
Journal Article
Research Support, U.S. Gov't, P.H.S.

Language

eng

PubMed ID

8510021

Citation

Castle, N A., and M T. Slawsky. "Characterization of 4-aminopyridine Block of the Transient Outward K+ Current in Adult Rat Ventricular Myocytes." The Journal of Pharmacology and Experimental Therapeutics, vol. 265, no. 3, 1993, pp. 1450-9.
Castle NA, Slawsky MT. Characterization of 4-aminopyridine block of the transient outward K+ current in adult rat ventricular myocytes. J Pharmacol Exp Ther. 1993;265(3):1450-9.
Castle, N. A., & Slawsky, M. T. (1993). Characterization of 4-aminopyridine block of the transient outward K+ current in adult rat ventricular myocytes. The Journal of Pharmacology and Experimental Therapeutics, 265(3), 1450-9.
Castle NA, Slawsky MT. Characterization of 4-aminopyridine Block of the Transient Outward K+ Current in Adult Rat Ventricular Myocytes. J Pharmacol Exp Ther. 1993;265(3):1450-9. PubMed PMID: 8510021.
* Article titles in AMA citation format should be in sentence-case
TY - JOUR T1 - Characterization of 4-aminopyridine block of the transient outward K+ current in adult rat ventricular myocytes. AU - Castle,N A, AU - Slawsky,M T, PY - 1993/6/1/pubmed PY - 1993/6/1/medline PY - 1993/6/1/entrez SP - 1450 EP - 9 JF - The Journal of pharmacology and experimental therapeutics JO - J Pharmacol Exp Ther VL - 265 IS - 3 N2 - The blocking action of 4-aminopyridine (4-AP) on the transient outward K+ current (ITO) in isolated rat ventricular myocytes was studied using the whole-cell configuration of the patch-clamp technique. 4-AP inhibition of ITO was concentration-dependent with half-maximal block occurring at 0.2 mM. At high concentrations (> 1 mM), 4-AP appeared to slow both the activation and inactivation phases of ITO. This resulted in a crossover phenomenon where in the presence of 4-AP the outward current was less than control at the beginning of a depolarizing pulse but crossed over during the pulse to become greater than control. Inhibition of ITO by 4-AP was voltage-dependent. Steady-state block of ITO by 4-AP was greatest at or near resting membrane potentials (i.e., -70 mV) but decreased with membrane depolarization. The voltage-dependence of block was steep and was well described by a Boltzmann relationship with a slope factor of approximately 4 mV. The midpoint potential for block was dependent on the concentration of 4-AP, being -41.6 +/- 0.4 mV (n = 9), -40.7 +/- 1.3 mV (n = 6), -34.0 +/- 1.6 mV (n = 5) and -30.1 +/- 0.2 (n = 15) at 0.3, 1, 3 and 10 mM, respectively. The midpoint potential for activation was -12.6 mV and was -46.9 mV for inactivation. The concentration-dependence of the voltage-dependence of 4-AP block can be explained by assuming that the sequential closed states through which the channel passes during activation exhibit successively lower affinities for 4-AP. Onset of ITO block by 4-AP was slow. The association (kON) and dissociation. (kOFF) rate constants for binding at -70 mV were: kON = 207 M-1 s-1 and kOFF = 0.090 s-1. The time constant for unblock (tau UNBLOCK) of ITO at 0 mV was independent of 4-AP concentration indicating that there was no binding of 4-AP at this potential. kOFF (1/tau UNBLOCK) at 0 mV was 2.4 s-1 which is approximately 25-fold faster then at -70 mV. The results suggest that 4-AP binds most strongly to closed channels with the inactivation gate open. The conformational changes that occur during channel opening induce a decrease in affinity for 4-AP so that when the channel is in the open state, 4-AP binding is at its weakest. The processes of 4-AP block and inactivation appear to be mutually exclusive. SN - 0022-3565 UR - https://www.unboundmedicine.com/medline/citation/8510021/Characterization_of_4_aminopyridine_block_of_the_transient_outward_K+_current_in_adult_rat_ventricular_myocytes_ L2 - https://jpet.aspetjournals.org/cgi/pmidlookup?view=long&pmid=8510021 DB - PRIME DP - Unbound Medicine ER -