Focal increases in [Ca2+](i) may account for apparent low Ca2+ sensitivity in swine carotid artery

Estimates of [Ca2+](i) sensitivity in intact smooth muscle are frequently obtained by measuring [Ca2+](i) with indicators such as aequorin or Fura-2. We investigated whether focal increases in [Ca2+](i) could impair such measures of [Ca2+](i) sensitivity. Stimulation of swine carotid artery with 10 mu M histamine increased aequorin estimated [Ca2+](i), Fura-2 estimated [Ca2+](i) and Ca2+ sensitivity without significantly altering the aequorin/Fura-2 ratio (an estimate of [Ca2+](i) homogeneity). Subsequent inhibition of Na+/Ca2+ exchange by replacement of Na+ in the PSS with choline(+) significantly increased aequorin-estimated [Ca2+](i) but only minimally increased Fura-2 estimated [Ca2+](i), myosin light chain (MLC) phosphorylation and force. This resulted in a large increase in the aequorin/Fura-2 ratio, suggesting an increase in [Ca2+] inhomogeneity. Addition of 100 mu M histamine to tissues in the choline(+) buffer initially increased both aequorin and Fura-2 estimated [Ca2+](i), but after 10 min exposure both of the [Ca2+](i) estimates declined to pre-histamine levels. Histamine addition significantly increased MLC phosphorylation and force, indicating increased Ca2+ sensitivity, but the aequorin/Fura-2 ratio remained elevated and unchanged from pre-histamine values. These data show that under certain conditions, aequorin and Fura-2 can yield widely differing estimates of [Ca2+](i) and thus can cause misleading assessments of Ca2+ sensitization mechanisms. These discrepancies may arise from inhomogeneous or focal increases in [Ca2+](i) which can be evaluated with the aequorin/Fura-2 ratio.