LONGITUDINAL DISTRIBUTION OF NA-ADRENOCEPTORS ON THE SARCOLEMMAL MEMBRANE OF FROG CARDIOMYOCYTES( AND CA2+ CHANNELS AND BETA)

1. The distribution of L-type Ca2+ and tetrodotoxin-sensitive Na+ chan nels and of beta-adrenergic receptors was examined in frog ventricular myocytes using tile whole-cell patch-clamp technique and a double cap illary for extracellular microperfusion. 2. Rod-shaped cells (250-300 mu m long) were sealed at both ends to two patch-clamp pipettes and po sitioned transversally at different positions between the mouths of tw o microcapillaries separated by a thin wall. A combination of nifedipi ne (1 mu M) and tetrodotoxin (0.3 mu M) (blocking solution) was added to one capillary in order to inhibit macroscopic Ca2+ and Na+ currents (I-Ca and I-Na, respectively) in the part of the cell exposed to this capillary. 3. Moving the cell in 10-20 mu m steps from the control ca pillary to the capillary containing the blocking solution induced step decreases in I-Ca and I-Na amplitudes. Complete block of both current s occurred when the entire cell was exposed to the blocking solution. 4. Each step decrease in current was due to the los of activity of the functional Ca2+ and Na+ channels present in the slice of sarcolemmal membrane newly exposed to the blocking solution. These step current ch anges allowed longitudinal mapping of current density for Ca2+ and Na channels on the sarcolemmal membrane. 5. Addition of a submaximal con centration of isoprenaline (10 nM) to the control capillary induced a local increase in I-Ca which enabled examination of the distribution o f functional beta-adrenergic receptors as well. 6. Our results demonst rate that Ca2+ and Na+ channels and beta-adrenergic receptors are equa lly and essentially uniformly distributed on the sarcolemmal membrane of frog ventricular myocytes.