Electrical and contractile consequences of Na⁺ or Ca²⁺ gradient reduction in cultured heart cells

The normal electrical and contractile activity of cultured neonatal rat ventricular cells is characterized and compared to activity seen in low [Ca²⁺]₀ and low [Na⁺]₀ solutions. In 0 Ca²⁺/0.5 mm EGTA solutions electrical activity is altered: the maximum diastolic potential (m.d.p.), maximum rate of rise (+ V ̇_max), and overshoot (o.s.) are reduced, while duration is increased. Low [Ca²⁺]₀ activity is insensitive to TTX and blocked by La³⁺. In low [Na⁺]₀ solutions electrical activity is generally absent; when present + V ̇_max and o.s. are decreased while duration is increased. Low [Na⁺]₀ activity is blocked by La³⁺. These data suggest the presence of one La³⁺-sensitive slow inward current channel. The absence of spontaneous electrical activity in low [Na⁺]₀ solutions suggests an inhibition of automaticity. To determine if this inhibition is due to a reduction of the Na⁺ gradient, drugs which raise [Na⁺]_i were examined. Both monensin (a Na⁺ ionophore) and ouabain inhibit the occurrence of spontaneous action potentials (cells respond to stimulation) indicating a dependence of pacemaker activity on a normal Na⁺ gradient. During Na⁺ gradient reduction, asynchronous subcellular contractile activity occurs independent of membrane potential fluctuation. This asynchronous activity is inhibited by La³⁺ or when Ca²⁺₀ is absent, but continues in the presence of verapamil (normal activity is blocked by all three conditions). The Na⁺Ca²⁺ exchange system is unaffected by verapamil but blocked by La³⁺, while both these drugs block the slow inward current. These data indicate that the Na⁺Ca²⁺ exchange system can directly supply Ca²⁺ (independent of electrical activity at the membrane) to intracellular sites that support contractile activity.