Local anesthetics depress the calcium current of rat sensory neurons in culture.

BACKGROUND

Local anesthetics are known to inhibit the voltage-gated sodium current (INa) of the nerve membrane, but it has not been fully studied whether anesthetic concentrations of local anesthetics depress the voltage-gated calcium current (ICa) of mammalian neurons. The effects of local anesthetics on ICa evoked in cultured rat dorsal root ganglion cells were studied.

METHODS

Whole cell patch clamp recordings were made from rat dorsal root ganglion cells cultured for 1-3 weeks. ICa was recorded using patch electrodes filled with Cs-aspartate in Na(+)-free external solution containing 5 mM-Ba2+. All drugs, including local anesthetics, were applied by miniperfusion from micropipettes by pressure ejection.

RESULTS

Tetracaine (300 microM) depressed the peak amplitudes of high voltage-activated (HVA)-ICa to 22.6 +/- 8.8% of control values (n = 14) without affecting the current-voltage relation. A tetracaine dose-response curve for HVA-ICa indicated an apparent dissociation constant of 79.5 microM. Tetracaine (30 microM) depressed nicardipine-sensitive HVA-I(Ca) (L-type) to 14.3 +/- 6.7% (n = 6), omega-conotoxin-sensitive HVA-ICa (N-type) to 81.6 +/- 9.6% (n = 7), and low voltage-activated (LVA)-ICa (T-type) to 65.1 +/- 11.1% (n = 6) of their respective controls. Local anesthetics other than tetracaine also depressed HVA-ICa but were of different potency; the rank sequence was dibucaine > tetracaine > bupivacaine >> procaine = lidocaine.

CONCLUSIONS

These results suggest that both HVA-ICa and LVA-ICa are depressed by tetracaine used at the concentrations required for spinal anesthesia and that the L-type Ca2+ channel among Ca2+ channel subtypes is the most susceptible to tetracaine. A good correlation between local anesthetic potencies to inhibit HVA-ICa and their anesthetic potencies implies that the inhibition of calcium influx through voltage-gated channels may contribute to spinal anesthetic mechanisms.