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 μM) was used to increase hydraulic conductivity (L(p)) 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 L(p) (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 a 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 Ca²⁺-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.