DILTIAZEM BLOCKS THE PH-INDUCED EXCITATION OF RAT NOCICEPTORS TOGETHER WITH
THEIR MECHANICAL AND ELECTRICAL EXCITABILITY, IN VITRO .
Wegner, Holger, Susanne Brehm, Peter W. Reeh, Hans W. Kreysel and Kay H.
Steen.
Universit[umlaut]atshautklinik und Poliklinik, der Universit[umlaut]at
Bonn, Sigmund-Freud-Str. 25, D-53105 Bonn, Germany, Institut f[umlaut]ur
Physiologie & experimentelle Pathophysiologie, der Universit[umlaut]at
Erlangen-N[umlaut]urnberg, Universit[umlaut]atsstr. 17, D-91054 Erlangen,
Germany.
APStracts 2:0246N, 1995.
SUMMARY AND CONCLUSIONS
1.) The effect of the calcium channel antagonist diltiazem on pH-induced
sustained nociceptor excitation was investigated in a rat skin-saphenous nerve
preparation, in vitro, where receptive fields of identified and isolated
single fibers were superfused at the corium side with controlled solutions to
assess their chemosensitivity. 2.) Unmyelinated mechano-heat sensitive
("polymodal") C-fiber terminals (n=78) were superfused with a CO 2 -saturated
synthetic interstitial fluid (CO 2 -SIF = pH 6.1). Fibers responding to this
acid pH condition (n=60; 77%) were further stimulated for at least 30 min and
additionally treated with diltiazem in various concentrations (10 -6 M to 10 -
3 M) during the middle 10 min-period . Usually only one concentration of
diltiazem was applied per fiber, although in some cases diltiazem was
repeatedly applied and in increasing concentrations. 3.) Diltiazem dose-
dependently and reversibly reduced the pH-induced sustained nociceptor
discharge to a significant degree or completely abolished it. With higher
concentrations, both the relative number of units affected and the average
amount of suppression were enhanced. The half-maximal blocking concentration
(IC 50) was estimated to 1.1 . 10 -4 M diltiazem, the half-maximal
concentration for gradual suppression of the pH response was 2 . 10 -5 M
diltiazem. 4.) Also, the delay of onset of the suppressive effect decreased
with higher diltiazem concentrations. Following diltiazem, a partial recovery
of the pH-induced discharge was achieved within 10 min depending on the degree
of suppression. 5.) Before, after and sometimes during the superfusion, the
mechanical (von Frey) thresholds were determined and found to be significantly
increased after partial wash-out of diltiazem (10 -4 and 10 -3 M; p<0.006 and
p<0.008, respectively). Following 10 -3 M diltiazem superfusion (and 10 min of
wash-out) the responsiveness to mechanical stimulation of the majority of the
fibers was still totally lost. 6.) Heat thresholds were still found to be
significantly increased following treatment with diltiazem at 10 -3 and 10 -4
M concentrations (and 10 min of wash-out), but appeared unchanged after wash-
out of lower concentrations. 7.) In five C-MH fibers, electrical stimulation
via a microelectrode placed in the receptive field was used to demonstrate a
diltiazem-dose-dependent (10 -5, 10 -4, 10 -3 M) progressive retardation of
the nerve conduction velocity and an increase of the electrical threshold.
Superfusion for 6 min of diltiazem 10 -5 M was sufficient to block axonal
conduction as well as mechanosensitivity which both recovered synchronously
during wash-out. 8.) It can be concluded from the results that the suppressive
effect of diltiazem on pH-induced nociceptor excitation can be explained by a
use-dependent axonal block, comparable to the action of local anesthetics and
affecting all modalities of sensory responsiveness. 9.) The findings provide
no indication that a transformed calcium channel specifically sensitive to
diltiazem is involved in pH-induced sustained nociceptor excitation.
Received 16 February 1995; accepted in final form 8 August 1995.
APS Manuscript Number J116-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 24 August 1995.