leiurotoxin-i and iberiotoxin

To elucidate the role of Ca(2+)-activated K(+) (K(Ca)) 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 K(Ca) 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 K(Ca) (BK) channel antagonists, iberiotoxin and paxilline enhanced the presynaptic ACh release and postsynaptic norepinephrine (NE) and epinephrine (Epi) release. Small-conductance K(Ca) (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.

Topics: Acetylcholine; Adrenal Medulla; Animals; Apamin; Catecholamines; Electric Stimulation; Indoles; Male; Neostigmine; Peptides; Potassium Channels, Calcium-Activated; Rats; Scorpion Venoms

The present study examined the possible role of voltage-dependent Ca++ channels (VDCs) and Ca(++)-activated K+ (KCa) channels in the regulation of resting tone of arteries from spontaneously hypertensive rats (SHR) at a prehypertensive stage. Differences in the effects of agents that interact with these channels were assessed in endothelium-denuded strips of femoral arteries isolated from 4-week-old SHR and age-matched normotensive Wistar-Kyoto rats (WKY). Systolic blood pressures at this age were not significantly different between SHR and WKY. The arterial strips from SHR maintained a myogenic tone in the resting state; that is the resting tone in the SHR artery was abolished when either the bathing solution was replaced with a Ca(++)-free solution or 10(-7) M nifedipine was added. Studies using 1- or 5-min pulse labeling of the arteries with 45Ca showed that the resting Ca++ influx was significantly increased in SHR when compared with WKY, and this increase in SHR was abolished by 10(-7) M nifedipine. In strips preloaded with fura-PE3, the addition of 3 x 10(-6) M verapamil to resting muscles decreased the resting cytosolic Ca++ level and caused a relaxation. These effects of verapamil were more evident in SHR than in WKY. The addition to the strips of charybdotoxin and iberiotoxin, blockers of large conductance KCa channels, caused a concentration-dependent contraction, which was significantly greater in SHR than in WKY.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: 3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; Age Factors; Animals; Apamin; Calcium; Calcium Channel Blockers; Calcium Channels; Charybdotoxin; Femoral Artery; Hypertension; Ion Channel Gating; Male; Membrane Potentials; Muscle, Smooth, Vascular; Nifedipine; Peptides; Potassium Channel Blockers; Potassium Channels; Rats; Rats, Inbred SHR; Rats, Inbred WKY; Scorpion Venoms; Verapamil

Despite recent progress in the molecular characterization of high-conductance Ca(2+)-activated K+ (maxi-K) channels, the molecular identities of intermediate conductance Ca(2+)-activated K+ channels, including that of mature erythrocytes, remains unknown. We have used various peptide toxins to characterize the intermediate conductance Ca(2+)-activated K+ channels (Gardos pathway) of human and rabbit red cells. With studies on K+ transport and on binding of 125I-charybdotoxin (ChTX) and 125I-kaliotoxin (KTX) binding in red cells, we provide evidence for the distinct nature of the red cell Gardos channel among described Ca(2+)-activated K+ channels based on (i) the characteristic inhibition and binding patterns produced by ChTX analogues, iberiotoxin (IbTX) and IbTX-like ChTX mutants, and KTX (1-37 and 1-38 variants); (ii) the presence of some properties heretofore attributed only to voltage-gated channels, including inhibition of K transport by margatoxin (MgTX) and by stichodactyla toxin (StK); (iii) and the ability of scyllatoxin (ScyTX) and apamin to displace bound 125I-charybdotoxin, a novel property for K+ channels. These unusual pharmacological characteristics suggest a unique structure for the red cell Gardos channel.

Topics: Animals; Calcium; Charybdotoxin; Erythrocytes; Humans; In Vitro Techniques; Ion Transport; Kinetics; Neurotoxins; Peptides; Point Mutation; Potassium; Potassium Channel Blockers; Potassium Channels; Rabbits; Rubidium; Scorpion Venoms