Calcium compartments in vascular smooth muscle cells as detected by aequorin signal

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Abstract
To examine whether cytosolic Ca2+ in smooth muscle cells distributes evenly, cytosolic Ca2+ levels were measured with two different Ca2+ indicators in the ferret isolated portal vein; a fluorescent indicator, fura‐PE3, that shows the average Ca2+ level, and a photoprotein, aequorin, that preferentially shows a high Ca2+ compartment. 2. A noradrenaline (10 μm)‐induced sustained contraction was associated with a sustained increase in the fura‐PE3 signal, or a transient increase followed by small sustained increase in the aequorin signal. A high K+‐induced contraction was associated with a sustained increase in both the fura‐PE3 and aequorin signals. 3. A second application of noradrenaline or high K+ induced reproducible contractions and fura‐PE3 signals. In contrast, the aequorin signal resulting from a second application of noradrenaline or high K+ was much smaller than the first signal. 4. Following a 13 h but not a 3 h resting period, the aequorin signal stimulated by noradrenaline or high K+ recovered, without any change in the contractile response. 5. In Ca2+‐free solution, high K+ was ineffective, whereas noradrenaline induced only a small aequorin signal and contraction compared to those obtained in the presence of external Ca2+. After the addition of Ca2+, the first application of noradrenaline induced a large aequorin signal and a large contraction, although a second application induced a much smaller aequorin signal accompanied by a large contraction. 6. These results suggest that high K+ and noradrenaline increase Ca2+ in at least two cytosolic compartments; a compartment that is coupled to the contractile mechanism (‘contractile’ Ca2+ compartment; major portion of cytoplasm containing contractile elements) and a compartment that is not coupled to contractile mechanisms (‘non‐contractile’ Ca2+ compartment; small sub‐membrane area that does not contain contractile elements). On stimulation, the Ca2+ level in the ‘contractile’ compartment may increase to a level high enough to stimulate myosin light chain kinase but not so high as to consume aequorin rapidly. In contrast, the Ca2+ level in the ‘non‐contractile’ compartment may increase so greatly that aequorin in this compartment is rapidly consumed. These two compartments may be separated by a diffusion barrier and, during a resting period, aequorin may slowly diffuse from the ‘contractile’ compartment to the ‘non‐contractile’ compartment and thus restore the full aequorin signal. An increase in Ca2+ in the ‘non‐contractile’ compartment seems to be dependent mainly on Ca2+ influx and partly on Ca2+ release.