Potassium channel blockade differentially affects the relative refractory period of frog afferent terminals and axons.
1. The effects of potassium channel blockade on afferent axons and terminal regions in frog dorsal roots and spinal cords, respectively, were investigated in vitro. 2. A condition-test (C-T) protocol was used to assess the population relative refractory period. Characteristics of main axons were evaluated by stimulation at the proximal end of transected dorsal roots (DR). Characteristics of terminal regions were tested by stimulation at the base of the dorsal horn (DH). 3. DH recovery of excitability was delayed by low concentrations of 4-aminopyridine (4-AP) and tetraethylammonium (TEA) alone or combined. The same treatments did not affect recovery to DR stimulation. 4. DH recovery of excitability was not delayed by solutions suppressing terminal calcium influx. 5. We conclude that sensitivity of the relative refractory period to potassium channel blocking agents differs between main axons and axon terminal regions. This may indicate differences between axon terminals and main axons in the mechanism of action potential repolarization. 6. We hypothesize that rapid action potential repolarization by pharmacologically sensitive potassium channels in presynaptic terminal regions keeps terminal action potentials short. Terminal action potential brevity would limit calcium influx, thus preventing terminal calcium overload but contributing to transmission failures at spinal synapses.
Tkacs
N C
NC
Department of Physiology, Loyola University Medical Center, Maywood, Illinois 60153.
Wurster
R D
RD
D015761:4-Aminopyridine / Q000494:pharmacology; D000200:Action Potentials / Q000187:drug effects; D000344:Afferent Pathways / Q000187:drug effects; D000818:Animals; D001340:Autonomic Nerve Block; D001369:Axons / Q000187:drug effects*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D005260:Female; D015640:Ion Channel Gating / Q000187:drug effects*; D008297:Male; D009411:Nerve Endings / Q000187:drug effects*; D015221:Potassium Channels / Q000187:drug effects* / Q000502:physiology; D011892:Rana catesbeiana; D013116:Spinal Cord / Q000187:drug effects* / Q000502:physiology; D013126:Spinal Nerve Roots / Q000187:drug effects* / Q000502:physiology; D019789:Tetraethylammonium; D013757:Tetraethylammonium Compounds / Q000494:pharmacology
D002121:Calcium Channel Blockers; D015221:Potassium Channels; D013757:Tetraethylammonium Compounds; D019789:Tetraethylammonium; D015761:4-Aminopyridine