Effects of verapamil on spinal anesthesia with local anesthetics

The primary mode of action of local anesthetics is through sodium channel and axonal conduction blockade. Local anesthetics also have extensive effects on presynaptic calcium channels that must function to stimulate the release of neurotransmitters. Thus, interference with calcium channel conductance may enhance spinal anesthesia with local anesthetics. The present study was designed to investigate the effects of the intrathecal calcium channel blocker, verapamil, on the spinal anesthesia from lidocaine and tetracaine. Male Sprague-Dawley rats were chronically implanted with lumbar intrathecal catheters. Tail-flick (TF) and mechanical paw pressure (MPP) tests were used to assess thermal and mechanical nociceptive threshold, respectively. Motor function was assessed using a modified Langerman's scale. Intrathecal lidocaine or tetracaine alone showed the prolongation of TF latency, the increase of MPP threshold, and the increase in motor function scale in a time- and dose-dependent manner. Although intrathecal verapamil alone demonstrated neither sensory nor motor block at the doses used (50-200 micrograms), the combination of lidocaine (20, 50, 100, or 200 micrograms) or tetracaine (10, 20, 50, or 100 micrograms) and verapamil (50 micrograms) produced the more potent and prolonged antinociception and motor block when compared with local anesthetics alone. We interpreted these results to indicate that the intrathecal calcium channel blocker, verapamil, potentiates spinal anesthesia with local anesthetics.