REGULATION OF NA+/K+ PUMP CURRENT IN THE HEART

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The Na/K pump is the primary metabolic engine for maintaining transmembrane ion gradients which are essential to excitability and contraction of the heart. Our recent data suggest ventricular cells have two functionally different types of Na/K pumps, referred to as type-h and type-l for their high and low affinity for the cardiac glycoside dihydro- ouabain (DHO). The type-l current is modulated by physiological variations in extracellular-K+ ([K]o) and extracellular-H+ ([H]o or pHo) whereas the type=h is not. Both are modulated by physiological variations in intracellular-Na+ ([Na]i), membrane voltage (phi/i) and extracellular- Na+ ([Na]o), though phi/i and [Na]o effects differ significantly between the two types of pumps. Autonomic input also regulates pump current. Gao, et al, 1992, showed stimulation of beta-receptors causes pump current to either increase or decrease depending on intracellular-Ca+2 ([Ca]i). The effects of beta-stimulation are phi/i-dependent, reversed by acetylcholine (ACh), and specifically target the type-l pumps. In contrast to these observations in ventricular cell, Cohen, et al, 1987, found DHO-blockade and [K]o-activation of the Na/K pumps in Purkinje cells corresponded closely to the type-h pumps only. These intriguing results suggest tissue specific, finely tuned regulation of Na/K pump current in the mammalian heart. Pathological states like ischemia, which result in an increase in [K]o and [H]o, will clearly affect the ventricular and Purkinje Na/K pumps differently and these differences will have a role in conduction abnormalities. Moreover, glycoside therapy will be toxic in Purkinje cells long before ventricular cells, and therapeutic doses will have different effects on the action potentials as well as [Na]i. The focus of this proposal is to understand the mechanisms responsible for regulating each of these two types of Na/K pumps in the heart. The whole cell patch clamp technique will be used on acutely isolated ventricular myocytes from the guinea pig heart. The type-h current is measured as that blocked by 5 microM DHO whereas total (type-h + type-l) current is blocked by 1 microM DHO. The effects of autonomic input on both types of pumps will be investigated with the goal of understanding each step in the cascades initiated by activation of a receptor and culminating in a change in Na/K pump current. Gao, et al, 1994a, show the response to beta-stimulation is through a phosphorylation step by phospho kinase A (PKA), but this cascade is not Ca+2-dependent. Thus, the mechanism of [Ca]i modulation of the beta-induced pump response is of particular interest. Moreover, differences in the two types of pumps' responses to [K]o, pHo and phi/i will be studied and related to differences in their responses to autonomic input.