Potentiating actions of lanthanum on ACh-induced cation current in guinea-pig ileal smooth muscle cells.

Potentiating actions of external lanthanum (La3+) on muscarinic receptor-activated nonselective cation current (Icat) were investigated in myocytes dissociated from the longitudinal muscle layer of guinea-pig ileum, with a whole-cell variant of the patch clamp technique. Icat was dissected from other membrane currents by loading Cs-aspartate into the cell. Application of submilimolar concentrations of La3+ following 300 microM ACh into the bath caused a dose-dependent increase in the amplitude of Icat. The apparent Kd value for this increase was 190 microM, with a cooperativity factor of 1.7. La(3+)-induced increase in Icat amplitude was not associated with either changes in the reversal potential of Icat or altered sensitivity of muscarinic receptor to ACh, and paralleled by the conductance increase of Icat, the maximum of which (Gmax) occurred at about 1 mM La3+. Voltage-jump experiments revealed that the rate of current relaxation at hyperpolarizing potentials was greatly reduced in the presence of La3+, and correspondingly the steady state activation curve shifted toward more negative potentials. Divalent cations such as Cd2+ or Ni2+, which have been known to block Icat, antagonized the augmentative effect of La3+ on Icat in a competitive fashion, suggesting that the site of their actions might be similar. Furthermore, single Icat activities induced by internal perfusion of GTP gamma S (100 microM) was also greatly enhanced by external addition of 1 mM La3+. Under current clamp conditions, 1 mM La3+ blocked spontaneous Ca2+ spike activities, but was almost without effect on the membrane depolarization induced by ACh. In contrast, milimolar concentrations of Cd2+ and Ni2+ abolished both Ca2+ spike activities and ACh-induced depolarization. Potential importance of La3+ as a tool to investigate the external Ca(2+)-dependence of Icat has been discussed.