POR1

Rho family GTPases are primary mediators of cytoskeletal reorganization, although they have also been reported to regulate cell secretion. Yet, the extent to which Rho family GTPases are activated by secretory stimuli in neural and neuroendocrine cells remains unknown. In bovine adrenal chromaffin cells, we found Rac1, but not Cdc42, to be rapidly and selectively activated by secretory stimuli using an assay selective for the activated GTPases. To examine effects of activated Rac1 on secretion, constitutively active mutants of Rac1 (Rac1-V12, Rac1-L61) were transiently expressed in adrenal chromaffin cells. These mutants facilitated secretory responses elicited from populations of intact and digitonin-permeabilized cells as well as from cells under whole cell patch clamp. A dominant negative Rac1 mutant (Rac1-N17) produced no effect on secretion. Expression of RhoGDI, a negative regulator of Rac1, inhibited secretory responses while overexpression of effectors of Rac1, notably, p21-activated kinase (Pak1) and actin depolymerization factor (ADF) promoted evoked secretion. In addition, expression of effector domain mutants of Rac1-V12 that exhibit reduced activation of the cytoskeletal regulators Pak1 and Partner of Rac1 (POR1) resulted in a loss of Rac1-V12-mediated enhancement of evoked secretion. These findings suggest that Rac1, in part, functions to modulate secretion through actions on the cytoskeleton. Consistent with this hypothesis, the actin modifying drugs phalloidin and jasplakinolide enhanced secretion, while latrunculin-A inhibited secretion and eliminated the secretory effects of Rac1-V12. In summary, Rac1 was activated by secretory stimuli and modulated the secretory pathway downstream of Ca2+ influx, partly through regulation of cytoskeletal organization.

Bradykinin mimics ischemic preconditioning by generating reactive oxygen species (ROS). To identify intermediate steps leading to ROS generation, rabbit cardiomyocytes were incubated in reduced MitoTracker Red that becomes fluorescent after exposure to ROS. Fluorescence intensity in treated cells was expressed as % of that in paired untreated cells. Bradykinin (500nM) caused 51+/-16% increase in ROS generation (p<0.001). Co-incubation with either bradykinin B2 receptor blocker HOE140 (5 micro M) or free radical scavenger N-(2-mercaptopropionyl) glycine (1mM) prevented this increase confirming the response was receptor-mediated and ROS were actually being measured. Closing mitochondrial KATP (mKATP)channels with 5-hydroxydecanoate (5HD, 1mM) prevented increased ROS generation. Bradykinin-induced ROS generation was blocked by L-NAME (200 micro M) implicating nitric oxide as an intermediate. Blockade of guanylyl cyclase with ODQ (10 micro M) aborted bradykinin-induced ROS generation, but not that from diazoxide, a direct opener of mKATP channels. The PKG blocker 8-Br-cGMPS (25 micro M) eliminated bradykinin's effect. Conversely, direct activation of PKG with 8-pCPT-cGMP (100 micro M) increased ROS generation (39+/-15%, p<0.004) similar to bradykinin. This increase was blocked by 5HD. Finally, the nitric oxide donor SNAP (1 micro M)increased ROS by 34+/-6%. This increase was also blocked by 5HD. In intact rabbit hearts bradykinin (400nM) decreased infarction from 30.5+/-3.0% of the risk zone in control hearts to 11.9+/-1.4% (p<0.01). This protection was aborted by either L-NAME (200 micro M) or ODQ (2 micro M)(35.4+/-5.7% and 30.4+/-3.0% infarction, resp., pNS vs control). Hence, bradykinin preconditions through receptor-mediated production of nitric oxide which activates guanylyl cyclase. The resulting cGMP activates PKG that opens mKATP. Subsequent release of ROS triggers cardioprotection.