Essential role for extracellular Ca(2+) in JNK activation by mechanical stretch in bladder smooth muscle cells

Mechanicalstretch has been implicated in phenotypic changes as an adaptiveresponse to stretch stress physically loaded in bladder smooth musclecells (BSMCs). To investigate stretch-induced signaling, we examinedthe mitogen-activated protein kinase (MAPK) family using rat primaryBSMCs. When BSMCs were subjected to sustained mechanical stretch usingcollagen-coated silicon membranes, activation of c-JunNH₂-terminal kinase (JNK) was most relevant among three subsets of MAPK family members: the activity was elevated from 5 minafter stretch and peaked at 10 min with an 11-fold increase. Activationof p38 was weak compared with that of JNK, and ERK was notactivated at all. JNK activation by mechanical stretch was totallydependent on extracellular Ca²⁺ and inhibited byGd³⁺, a blocker of stretch-activated (SA) ion channels.Nifedipine and verapamil, inhibitors for voltage-dependentCa²⁺ channels, had no effect on this JNK activation.Moreover, none of the inhibitors pertussis toxin, genistein,wortmannin, or calphostin C affected stretch-induced JNK activation,indicating that G protein-coupled and tyrosine kinase receptors areunlikely to be involved in this JNK activation. On the other hand, W-7,a calmodulin inhibitor, and cyclosporin A, a calcineurin inhibitor,prevented JNK activation by stretch. These results suggest a novelpathway for stretch-induced activation of JNK in BSMCs: mechanicalstretch evokes Ca²⁺ influx via Gd³⁺-sensitiveSA Ca²⁺ channels, resulting in JNK activation underregulation in part by calmodulin and calcineurin.