Effect of calcium on the vasoactive response of arterioles to light during fluorescent intravital microscopy of the tibialis anterior muscle of the hamster.

Arterioles may undergo a transient vasoactive response when exposed to light during fluorescent intravital microscopy. We hypothesized that the type and frequency of the vasoactive responses by arterioles to light is calcium dependent. In order to test this hypothesis we quantitated the type and frequency of vasoactive responses by arterioles to light in the presence and absence of calcium within the suffusate bathing the muscle. In addition, we determined whether the presence or absence of calcium also influenced the reactivity of these arterioles to adenosine and norepinephrine. In separate experiments, we determined the effect of increases in suffusate calcium concentration on the resting diameter of arterioles. The concentration of calcium in the suffusate significantly influenced the frequency of light-induced vasoactivity of the arterioles of the tibialis anterior muscle: a normal suffusate calcium concentration of 2 x 10(-3) M was associated with an incidence of light-induced vasoreactivity of 77% but decreased significantly to 58% when calcium was removed from the suffusate. Although the frequency of the vasoactive response to light was different for the two experimental conditions, the type of vasoactive response, predominately vasomotion, was similar. The vasoactive responses of arterioles to adenosine and norepinephrine were similar in the presence and absence of calcium in the suffusate. However, the concentration of calcium in the suffusate did significantly influence resting arteriolar diameters: 2 x 10(-3) M calcium CaCl2 caused a mean decrease in the arteriolar diameter of 11.6 (+/- 3.0)%, 4 x 10(-3) M CaCl2 caused a mean decrease of 41.2 (+/- 12)%, and 8 x 10(-3) M CaCl2 caused a mean decrease of 55.4 (+/- 14.4)%. These results show that the concentration of calcium in the suffusate bathing the tibialis anterior muscle during fluorescent intravital microscopy significantly influences the vasoactive response of arterioles to light. Further investigation of the mechanisms of light-induced effects on microcirculation during fluorescent intravital microscopy will become important as this technique is used more widely in the study of the microcirculation of solid tissues and organs.