Intracellular cations modulate noradrenaline‐stimulated calcium entry into smooth muscle cells of rat portal vein.

1. The action of noradrenaline (NA, 10 microM) was studied in single patch‐clamped smooth muscle cells of rat portal vein where free internal Ca2+ concentration in the cell (Ca2+i) was estimated using the emission from the dye Indo‐1. 2. In the presence of 50 microM D600, a blocker of voltage‐dependent Ca2+ channels, NA applied to the cell evoked an initial peak in Ca2+i followed by a smaller sustained rise. The initial rise in Ca2+i was associated with the activation of Ca(2+)‐dependent Cl‐ channels. 3. The maintained rise in Ca2+i induced by NA was enhanced by increasing the external Ca2+ concentration and was abolished in Ca(2+)‐free solution. The transient rise was resistant to the absence of external Ca2+. 4. Following the transient rise in Ca2+i induced by NA, the mechanisms extruding and/or sequestering cytoplasmic Ca2+ were stimulated. This stimulation was measured during the sustained rise in Cai and was maintained for a few seconds after NA was removed. 5. Unlike the transient rise in Ca2+i, the sustained rise in Ca2+i 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 Ca2+i. 7. The sustained increase in Ca2+i 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 Ca2+i 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 Ca2+i 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 Ca2+i induced by NA was modulated by the intracellular cations.