A-type potassium current in retinal arteriolar smooth muscle cells.
OBJECTIVE
By their control of membrane potential and intracellular free Ca(2+) ([Ca(2+)](i)), K(+) currents are pivotal in the regulation of arterial smooth muscle tone. The goal of the present study was to identify and characterize the A-type K(+) current in retinal microvascular smooth muscle (MVSM) and to examine its role in modulating membrane potential and cellular contractility.


METHODS
Whole-cell perforated patch-clamp recordings were made from MVSM cells within intact isolated arteriolar segments. Before patch-clamping, retinal arterioles were anchored in the physiological recording bath and perfused with an enzyme cocktail to remove surface basal lamina and to uncouple electrically the endothelial cells from the overlying MVSM cells.


RESULTS
K(+) currents were activated by depolarizing steps from -80 to +100 mV in 20-mV increments. A dominant, noninactivating current was elicited by depolarization to potentials positive of -50 mV. Inhibition of this current by 100 nM of the Ca(2+)-activated K(+) channel blocker, Penitrem A, revealed a rapidly inactivating K(+) current that resembled an A-type current. The A-type current was insensitive to tetraethylammonium (TEA) at 1 mM, but was partially suppressed by higher concentrations (10 mM). 4-Aminopyridine (10 mM; 4-AP) completely blocked the A-type current. The 4-AP-sensitive transient current was activated at a potential of -60 mV with peak current densities averaging 29.7 +/- 5.68 pA/pF at +60 mV. The voltage of half-inactivation was -28.3 +/- 1.9 mV, and the time constant for recovery from inactivation at +60 mV was 118.7 +/- 7.9 ms. Under current-clamp conditions 4-AP depolarized the membrane potential by approximately 3 to 4 mV and triggered small contractions and relaxations of individual MVSM cells within the walls of the arterioles.


CONCLUSIONS
A-type current is the major voltage-dependent K(+) current in retinal MVSM and appears to play a physiological role in suppressing cell excitability and contractility.
McGahon
Mary K
MK
Centre of Vision Sciences, The Queen's University of Belfast, Institute of Clinical Sciences, The Royal Victoria Hospital, Grosvenor Road, Belfast BT12 6BA, Northern Ireland.
Dawicki
Jennine M
JM
Scholfield
C Norman
CN
McGeown
J Graham
JG
Curtis
Tim M
TM
D015761:4-Aminopyridine / Q000494:pharmacology; D000818:Animals; D001160:Arterioles; D004594:Electrophysiology; D004730:Endothelium, Vascular; D008297:Male; D008564:Membrane Potentials; D009131:Muscle, Smooth, Vascular / Q000502:physiology*; D009183:Mycotoxins / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D026902:Potassium Channel Blockers / Q000494:pharmacology; D024681:Potassium Channels, Calcium-Activated / Q000187:drug effects; D024642:Potassium Channels, Voltage-Gated / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012161:Retinal Artery / Q000187:drug effects / Q000502:physiology* / Q000648:ultrastructure; D019789:Tetraethylammonium / Q000494:pharmacology
D009183:Mycotoxins; D026902:Potassium Channel Blockers; D024681:Potassium Channels, Calcium-Activated; D024642:Potassium Channels, Voltage-Gated; C003626:tremortin; D019789:Tetraethylammonium; D015761:4-Aminopyridine