cGMP reduces the sarcoplasmic reticulum Ca2+ loading in airway smooth muscle cells: a putative mechanism in the regulation of Ca2+ by cGMP

Ca2+ and cGMP have opposite roles in many physiological processes likely due to a complex negative feedback regulation between them. Examples of opposite functions induced by Ca2+ and cGMP are smooth muscle contraction and relaxation, respectively. A main Ca2+ storage involved in contraction is sarcoplasmic reticulum (SR); nevertheless, the role of cGMP in the regulation of SR-Ca2+ has not been completely understood. To evaluate this role, intracellular Ca2+ concentration ([Ca2+]i) was determinated by a ratiometric method in isolated myocytes from bovine trachea incubated with Fura-2/AM. The release of Ca2+ from SR induced by caffeine was transient, whereas caffeine withdrawal was followed by a [Ca2+]i undershoot. Caffeine-induced Ca2+ transient peak and [Ca2+]i undershoot after caffeine were reproducible in the same cell. Dibutyryl cGMP (db-cGMP) blocked the [Ca2+]i undershoot and reduced the subsequent caffeine peak (SR-Ca2+ loading). Both, the opening of SR channels with ryanodine (10 μM) and the blockade of SR-Ca2+ ATPase with cyclopiazonic acid inhibited the [Ca2+]i undershoot as well as the SR-Ca2+ loading. The addition of db-cGMP to ryanodine (10 μM) incubated cells partially restored the SR-Ca2+ loading. Cyclic GMP enhanced [Ca2+]i undershoot induced by the blockade of ryanodine channels with 50 μM ryanodine. In conclusion, the reduction of SR-Ca2+ content in airway smooth muscle induced by cGMP can be explained by the combination of SR-Ca2+ loading and the simultaneous release of SR-Ca2+. The reduction of SR-Ca2+ content induced by cGMP might be a putative mechanism limiting releasable Ca2+ in response to a particular stimulus.