EFFECTS OF SOME DIVALENT CATIONS ON SYMPATHETIC GANGLION FUNCTION

¹ Department of Pharmacology, University of Oregon Medical School, Portland, Oregon

The effects of Ni⁺⁺, Co⁺⁺, Zn⁺⁺, Mg⁺⁺ and Sr⁺⁺ on synaptic transmission and electrical properties of the ganglion cell were examined by extracellular and intracellular recording in isolated bullfrog sympathetic ganglia. CaCl₂ (1.8 mM) was always present in the Ringer's solution, to which the test divalent cation was then added. The linear parts of the concentration-transmission block curves were in the ranges: 0.3 to 1 mM (Co⁺⁺, Zn⁺⁺, Ni⁺⁺) and 3 to 10 mM (Mg⁺⁺, Sr⁺⁺). Mg⁺⁺ block was not purely synaptic, as it also depressed preganglionic axonal conduction. Intracellular recordings showed that orthodromic transmission block could be reversed by increasing stimulus repetition rates 3- or 10-fold. Antidromic spike amplitudes were significantly reduced by 10 and 26 mM Mg⁺⁺ and 10 mM Co⁺⁺, but significantly increased by 1 mM Co⁺⁺ and 10 mM Sr⁺⁺. All tested concentrations of Co⁺⁺, Mg⁺⁺ and Sr⁺⁺ raised the current threshold for direct stimulation, and all but Sr⁺⁺ reduced resting membrane potential. Sr⁺⁺ was also unique in causing a high-frequency discharge of miniature synaptic potentials. The data emphasize that potency order for transmission block parallels that for depression of overshoot in the "Ca⁺⁺ membrane" of barnacle muscle and that Sr⁺⁺ and Mg⁺⁺, despite equivalent synaptic blocking potencies, differ strikingly in their effects on electrical properties of the ganglion cell and on the frequency of miniature synaptic potentials.