Ca2+-induced aortic contraction in normal K+ after exposure to Ca2+- and Mg2+ -free solution

Guinea pig and rabbit aortic rings which have been incubated in a Ca2+- and Mg2+ -free solution containing ethylene glycol bis-(beta-aminoethyl ether) N,N,N'N'-tetraacetic acid (EGTA) responded to addition of Ca2+ in a similar Ca2+ - and Mg2+ -free solution without EGTA by increasing tension gradually at first and then more sharply at Ca2+ concentrations above 2 mM. Prior addition of D 600 (10 or 50 microM) inhibited contractions to low Ca2+ concentrations, but only partially attenuated those to high Ca2+ concentrations. The resulting contraction is thought to represent Ca2+ entry from both voltage-dependent (D 600-sensitive) Ca2+ channels and Na+ -Ca2+ exchange (D 600-insensitive).