Calcium dependence of C-type natriuretic peptide-formed fast K⁺ channel

The lipid bilayer technique was used to characterize the Ca²⁺ dependence of a fast K⁺ channel formed by a synthetic 17-amino acid segment [OαCNP-39-(1-17)] of a 39-amino acid C-type natriuretic peptide (OαCNP-39) found in platypus (Ornithorhynchus anatinus) venom (OαV). The OαCNP-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⁺](cis) was >10^-4 M 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 x 10^-6 M cis Ca²⁺ to 0.73 ± 0.17 (n = 22) at 10^-3 M cis Ca²⁺. Channel openings to the maximum conductance of 38 pS were rapidly and reversibly activated when [Ca²⁺](cis), but not trans Ca²⁺ 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 Ca²⁺. K⁺ channels did not open when cis Mg²⁺ or Sr²⁺ concentrations were increased from zero to 10^-3 M or when [Ca²⁺](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 Ca²⁺, respectively. This dependence of the channel on high [Ca²⁺](cis) suggests 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.