Role of Ca2+-activated K+ channels in catecholamine release from in vivo rat adrenal medulla

To elucidate the role of Ca2+-activated K+ (KCa) channels in the presynaptic acetylcholine (ACh) release from splanchnic nerve endings and the postsynaptic catecholamine release from chromaffin cells, we applied microdialysis technique to the left adrenal medulla of anesthetized rats and investigated the effects of local administration of KCa channel antagonists through dialysis probes on the release of ACh and/or catecholamine, induced by electrical stimulation of splanchnic nerves or local administration of ACh through the dialysis probes. Nerve stimulation-induced release: in the presence of a cholinesterase inhibitor, neostigmine, large-conductance KCa (BK) channel antagonists, iberiotoxin and paxilline enhanced the presynaptic ACh release and postsynaptic norepinephrine (NE) and epinephrine (Epi) release. Small-conductance KCa (SK) channel antagonists, apamin and scyllatoxin enhanced the Epi release without any changes in ACh or NE release. In the absence of neostigmine, ACh release was not detected. Iberiotoxin and paxilline enhanced NE and Epi release. Apamin and scyllatoxin had no effect on NE or Epi release. Exogenous ACh-induced release: iberiotoxin and paxilline enhanced the Epi release, but had no effect on the NE release. Apamin and scyllatoxin enhanced both NE and Epi release. In conclusion, BK channels on splanchnic nerve endings play an inhibitory role in the physiological catecholamine release from adrenal medulla by limiting presynaptic ACh release while SK channels do not. BK channels on Epi-storing cells may play an inhibitory role in nerve stimulation-induced Epi release. SK channels on NE- and Epi-storing cells play a minor role in nerve stimulation-induced catecholamine release.