Cellular Ca2+ monitored by aequorin in adenosine-mediated smooth muscle relaxation

We studied the effect of adenosine on cytoplasmic ionized calcium-force relationships in vascular smooth muscle (VSM) and determined the dose dependence of the observed effects. The bioluminescent protein aequorin was used as an index of cytoplasmic ionized calcium and was chemically loaded into ferret portal vein strips. The VSM strips were contracted with 33 mM potassium (K+), 5 X 10(-6) M phenylephrine (PE), or electrical stimulation. Force and aequorin light, i.e., cytoplasmic ionized calcium, were simultaneously recorded. Adenosine pretreatment (3.7 X 10(-6) M) reduced both force and light responses in contractures with K+, PE, or electrical stimulation. In contrast, the addition of adenosine during PE or K+ contractions decreased force without a change in light. Dose-response curves for the effects of adenosine on K+ contractures indicated that at low doses adenosine decreases force and cytoplasmic ionized calcium but that at high concentrations (greater than 3.7 X 10(-6) M) adenosine increases light and apparently relaxes VSM by desensitizing the myofilaments to cytoplasmic ionized calcium.