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An analysis of the influence of cholinergic agonist and antagonist movement across frog tadpole skin reveals that acetylcholine (ACh) induces the opening of nonselective cation channels at the apical membrane of frog tadpole skin. The apical amiloride/ACh induction of short-circuit current in tadpole skin signifies attachment with an apical receptor. The properties of acetylcholine responsive receptors are similar to those of muscarinic and nicotinic receptors.

ATP activates an apical-to-basolateral nonselective cation current across the skin of larval bullfrogs (Rana catesbeiana) with similarities to currents carried by some P2X receptors. A functional P2X receptor was cloned from tadpole skin RNA that encodes a 409-amino acid protein with highest protein homology to cP[2X.sub.8]. RT-PCR showed that this transcript was found in skin, heart, eye, brain, and skeletal muscle of tadpoles but not in skin, brain, or heart of adults. After transcribed RNA from this clone was injected into Xenopus oocytes, application of ATP activated a transient current similar to other P2X receptors and the ATP-activated transient in short-circuit current ([I.sub.sc]) across intact skin. The agonists 2-methylthio-ATP and adenosine-5'-O-(thiotriphoshate) also activated transient currents. [Alpha],[Beta]-Methylene-ATP and ADP were poor agonists of this receptor. Suramin and pyridoxal phosphate 6-azophenyl-2',4'-disulfonic acid tetrasodium (PPADS) were potent antagonists, and PPADS showed an irreversible blockade of this receptor to agonist activation. Under external [Na.sup.+]-free, [Ca.sup.2+]/[Mg.sup.2+]-free conditions (N-methyl-D-glucamine replacement, 0.5 mM EGTA), ATP activated a steadily increasing inward current. Fluorescence microscopy showed that propidium was entering the cells, suggesting that a relatively large pore size was formed under zero divalent conditions. This clone has some characteristics consistent with previously described ATP-activated [I.sub.sc] in the tadpole skin. Because the clone is not found in adult skin, it may have some exclusive role in the tadpole such as sensory reception by the skin or triggering apoptosis at metamorphosis. tadpole; P2X receptor clone; purinergic; amiloride; epithelium

Studies Ca2+ channels incorporated from partially purified boar sperm plasma membranes, using planar bilayer recordings. Voltage and time dependence of divalent selective channels; Single channel activity in the presence of DHP; Inhibition of channel activity of BAY K 8644 enantiomer (R+).