Calcium dependence of C-type natriuretic peptide-formed fast K(+) channel.
The lipid bilayer technique was used to characterize the Ca(2+) dependence of a fast K(+) channel formed by a synthetic 17-amino acid segment [OaCNP-39-(1-17)] of a 39-amino acid C-type natriuretic peptide (OaCNP-39) found in platypus (Ornithorhynchus anatinus) venom (OaV). The OaCNP-39-(1-17)-formed K(+) channel was reversibly dependent on 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-buffered cis (cytoplasmic) Ca(2+) concentration ([Ca(2+)](cis)). The channel was fully active when [Ca(2+)](cis) was >10(-4) M and trans (luminal) Ca(2+) concentration was 1.0 mM, but not at low [Ca(2+)](cis). The open probability of single channels increased from zero at 1 x 10(-6) M cis Ca(2+) to 0.73 +/- 0.17 (n = 22) at 10(-3) M cis Ca(2+). Channel openings to the maximum conductance of 38 pS were rapidly and reversibly activated when [Ca(2+)](cis), but not trans Ca(2+) concentration (n = 5), was increased to >5 x 10(-4) M (n = 14). Channel openings to the submaximal conductance of 10.5 pS were dominant at >/=5 x 10(-4) M Ca(2+). K(+) channels did not open when cis Mg(2+) or Sr(2+) concentrations were increased from zero to 10(-3) M or when [Ca(2+)](cis) was maintained at 10(-6) M (n = 3 and 2). The Hill coefficient and the inhibition constant were 1 and 0.8 x 10(-4) M cis Ca(2+), respectively. This dependence of the channel on high [Ca(2+)](cis) suggests that it may become active under 1) physiological conditions where Ca(2+) levels are high, e.g., during cardiac and skeletal muscle contractions, and 2) pathological conditions that lead to a Ca(2+) overload, e.g., ischemic heart and muscle fatigue. The channel could modify a cascade of physiological functions that are dependent on the Ca(2+)-activated K(+) channels, e.g., vasodilation and salt secretion.
Membrane Transport Group, Department of Chemistry, The Faculties, The Australian National University, Canberra City, Australian Capital Territory 0200, Australia. joseph.kourie@anu.edu.au
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D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D001665:Binding Sites / Q000502:physiology; D002118:Calcium / Q000378:metabolism / Q000502:physiology*; D004553:Electric Conductivity; D036341:Intercellular Signaling Peptides and Proteins; D007700:Kinetics; D008051:Lipid Bilayers; D008274:Magnesium / Q000494:pharmacology; D009994:Osmolar Concentration; D010446:Peptide Fragments / Q000502:physiology; D010455:Peptides / Q000502:physiology*; D010987:Platypus; D015221:Potassium Channels / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D013237:Stereoisomerism; D013997:Time Factors
D036341:Intercellular Signaling Peptides and Proteins; D008051:Lipid Bilayers; D010446:Peptide Fragments; D010455:Peptides; D015221:Potassium Channels; C114236:ovCNP-39 peptide; D000255:Adenosine Triphosphate; D008274:Magnesium; D002118:Calcium