The lipid bilayer technique was used to characterize the Ca2+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) Ca2+concentration ([Ca2+]cis). The channel was fully active when [Ca2+]ciswas >10−4M and trans (luminal) Ca2+concentration was 1.0 mM, but not at low [Ca2+]cis. The open probability of single channels increased from zero at 1 × 10−6M cisCa2+to 0.73 ± 0.17 ( n = 22) at 10−3M cisCa2+. Channel openings to the maximum conductance of 38 pS were rapidly and reversibly activated when [Ca2+]cis, but not transCa2+concentration ( n = 5), was increased to >5 × 10−4M ( n = 14). Channel openings to the submaximal conductance of 10.5 pS were dominant at ≥5 × 10−4M Ca2+. K+channels did not open when cisMg2+or Sr2+concentrations were increased from zero to 10−3M or when [Ca2+]ciswas maintained at 10−6M ( n = 3 and 2). The Hill coefficient and the inhibition constant were 1 and 0.8 × 10−4M cisCa2+, respectively. This dependence of the channel on high [Ca2+]cissuggests that it may become active under 1) physiological conditions where Ca2+levels are high, e.g., during cardiac and skeletal muscle contractions, and 2) pathological conditions that lead to a Ca2+overload, e.g., ischemic heart and muscle fatigue. The channel could modify a cascade of physiological functions that are dependent on the Ca2+-activated K+channels, e.g., vasodilation and salt secretion.
D-Amino acid residue in the C-type natriuretic peptide from the venom of the mammal,Ornithorhynchus anatinus, the Australian platypus
