Fig. 4.

Influence of membrane potential on I_max (a) and K_0.5 (b) for histamine-induced currents. PMAT-expressing oocytes were exposed to increasing concentrations of histamine at varying testing membrane potentials over the range of −10 to −150 mV. The substrate-dependent currents were subjected to kinetic analysis according to the Michaelis-Menten equation, and the kinetic parameters, I_max and K_0.5, were calculated. The data show that hyperpolarization increases the magnitude of the maximal currents induced by histamine. This effect is apparent at pH 7.5 and 6, both in the presence (NaCl) and absence (NMDG chloride) of Na⁺. The data also show that hyperpolarization has very little effect on the affinity of the transporter for histamine.