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

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Abstract
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.