Differential actions of cAMP on endothelial [Ca2+]i and permeability in microvessels exposed to ATP

We investigated the hypothesis that one of the actions of adenosine 3',5'-cyclic monophosphate (cAMP) to attenuate agonist-induced increase in microvessel permeability is to reduce the initial increase in endothelial cell cytoplasmic Ca2+ concentration ([Ca2+]i). ATP (10 microM) was used to increase hydraulic conductivity (Lp) and [Ca2+]i in venular microvessel in frog mesentery. The cAMP analogue, 8-bromo-cAMP (8-BrcAMP, 2 mM) significantly attenuated the initial increase in Lp (from a peak increase of 7.1 +/- 2.4 times control with ATP alone to 1.2 +/- 0.34 times control with ATP and cAMP). In contrast, cAMP did not significantly change the magnitude or time course of the biphasic increase in [Ca2+]i, which increased from 54 +/- 5 nM to peak of 187 +/- 48 nM with ATP alone, and from 55 +/- 8 nM to 190 +/- 28 nM with ATP and cAMP. Thus the action of cAMP to attenuate microvessel permeability increase in the presence of ATP does not involve modification of the initial Ca2+ transient in endothelial cells. Our results enable alternative hypotheses, such as the suggestion that cAMP might modulate a Ca(2+)-calmodulin-dependent actin-myosin cascade, to be investigated without the added complications of having to account for the modulation of initial changes in [Ca2+]i. Our data also demonstrate, for the first time in the endothelial cells of intact microvessels, that an increase in [Ca2+]i is not always associated with an increase in microvessel permeability.