Effect of cryptands on cardiac Purkinje fibres

Cryptands are possibly useful synthetical models for passive ion carriers in biomembranes. Five mainly Na+ and K+ complex forming cryptands were studied in cardiac Purkinje fibres by means of standard microelectrode technique. Even at high concentrations the cryptands 22, 221 and 222 showed no effect. Possibly they are not lipophilic enough to accumulate within the membrane. The more lipophilic cryptands 222B and 222D caused a decrease of overshoot and maximal rate of rise (Vmax), a pronounced loss of plateau and a shortening of the action potential, while the resting potential was only slightly depolarized. 222B made some fibres inexcitable at normal resting potentials; only by increase in stimulation voltage a regular response was obtained. After about 20 min both cryptands (5 x 10(-5) mol/l) depolarized the fibres leading to irreversible inexcitability. The availability of the fast sodium current ("h infinity"-curve) was measured using Weidmann's (1955) technique. With 222B the h infinity-curve-showed a shift in the hyperpolarizing direction, and the maximal Vmax was diminished.