Longitudinal distribution of Na + and Ca 2+ channels and β‐adrenoceptors on the sarcolemmal membrane of frog cardiomyocytes

1 The distribution of L‐type Ca 2+ and tetrodotoxin‐sensitive Na + channels and of β‐adrenergic receptors was examined in frog ventricular myocytes using the whole‐cell patch‐clamp technique and a double capillary for extracellular microperfusion. 2 Rod‐shaped cells (250–300 μm long) were sealed at both ends to two patch‐clamp pipettes and positioned transversally at different positions between the mouths of two micro‐capillaries separated by a thin wall. A combination of nifedipine (1 μ m ) and tetrodotoxin (0.3 μ m ) (blocking solution) was added to one capillary in order to inhibit macroscopic Ca 2+ 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 μm steps from the control capillary to the capillary containing the blocking solution induced step decreases in I Ca and I Na amplitudes. Complete block of both currents occurred when the entire cell was exposed to the blocking solution. 4 Each step decrease in current was due to the loss of activity of the functional Ca 2+ and Na + channels present in the slice of sarcolemmal membrane newly exposed to the blocking solution. These step current changes allowed longitudinal mapping of current density for Ca 2+ and Na + channels on the sarcolemmal membrane. 5 Addition of a submaximal concentration of isoprenaline (10 n m ) to the control capillary induced a local increase in I Ca which enabled examination of the distribution of functional β‐adrenergic receptors as well. 6 Our results demonstrate that Ca 2+ and Na + channels and β‐adrenergic receptors are equally and essentially uniformly distributed on the sarcolemmal membrane of frog ventricular myocytes.