Objectives: 

Depolarization-evoked sustained arterial contraction can be mediated by extracellular Ca2+ influx through L-type voltage-gated Ca2+ channels (VGCCs) and RhoA/Rho-associated kinase (ROCK)-dependent Ca2+ sensitization of the contractile machinery. VGCCs activation can also trigger an ion-independent metabotropic pathway that involves G-protein/phospholipase C (PLC) activation, inositol 1,4,5-trisphosphate synthesis, and Ca2+ release from the sarcoplasmic reticulum (SR) (Calcium Channel-Induced Ca2+ Release or CCICR). We have studied the functional role of CCICR and the inter-relations between VGCCs and RhoA/ROCK activation.

Materials: 

Rat arterial rings contraction was measured with a small-vessel myograph. RhoA activity was determined using a G-LISA RhoA activation assay kit. Statistical significance was estimated using the Student's t Test. Values of p<0.05 were considered significant.

Results: 

Arterial rings stimulated by a depolarizing solution showed an isometric contraction with an initial rapid (phasic) component followed by a sustained (tonic) component. The tonic component was selectively reduced by cyclopiazonic acid, U73122, and GDP&#946;S that inhibit SR Ca2+ ATPase, PLC and G proteins, respectively. In precontracted arteries, RhoA activity and the sustained component of the contraction were reduced in the presence of GDP&#946;S and Y27632, a ROCK inhibitor.

Conclusions: 

These data indicate that VGCCs have a much broader role in the regulation of vascular smooth muscle contraction than previously thought, as CCICR is necessary for sustained arterial contraction. VGCCs not only mediate the Ca2+ influx that triggers arterial contraction but also metabotropically regulate SR Ca2+ release and RhoA activation to sensitize the contractile machinery and the maintenance of arterial contraction.