INTRACELLULAR CATIONS MODULATE NORADRENALINE-STIMULATED CALCIUM ENTRY INTO SMOOTH-MUSCLE CELLS OF RAT PORTAL-VEIN

1. The action of noradrenaline (NA, 10 muM) was studied in single patch-clamped smooth muscle cells of rat portal vein where free internal Ca2+ concentration in the cell (Ca(i)2+) was estimated using the emission from the dye Indo-1. 2. In the presence of 50 muM D600, a blocker of voltage-dependent Ca2+ channels, NA applied to the cell evoked an initial peak in Ca2+ followed by a smaller sustained rise. The initial rise in Ca(i)2+ was associated with the activation of Ca2+-dependent Cl-channels. 3. The maintained rise in Ca(i)2+ induced by NA was enhanced by increasing the external Ca2+ concentration and was abolished in Ca2+-free solution. The transient rise was resistant to the absence of external Ca2+. 4. Following the transient rise in Ca(i)2+ induced by NA, the mechanisms extruding and/or sequestering cytoplasmic Ca2+ were stimulated. This stimulation was measured during the sustained rise in Ca(i) and was maintained for a few seconds after NA was removed. 5. Unlike the transient rise in Ca(i)2+, the sustained rise in Ca(i)2+ produced by NA was affected by changing the cell membrane potential. 6. Changing the Na+ gradient showed that the Na+-Ca2+ exchange was not involved in the sustained rise in Ca(i)2+. 7. The sustained increase in Ca(i)2+ produced by NA was modulated by intracellular cations. This phase could be observed with 130 mM Na+ or 130 mM K+ in the pipette solution, but was severely reduced when the only cation in the intracellular solution was Cs+ and abolished with NMDG (N-methyl-D-glucamine) or TEA. However, inclusion of only 10 mM Na+ or 50 mM K+ in the pipette solution was sufficient to obtain a sustained rise in Ca(i)2+ of maximal amplitude. similar to that obtained with 130 mM Na(i)+ or 130 mM K(i)+ during NA application. 8. In portal vein smooth muscle cells, NA induced a two-phase increase in Ca(i)2+ similar to the two phases which have been previously observed upon muscarinic receptor activation by carbachol in intestinal smooth muscle, The transient rise was due to Ca2+ store release whereas the sustained rise was due to an increased Ca2+ entry into the cell down its electrochemical gradient, but not through voltage-dependent Ca2+ channels. The Ca2+ permeability pathway involved in the sustained rise in Ca(i)2+ induced by NA was modulated by the intracellular cations.