Functional Bias of Contractile Control in Mouse Resistance Arteries
BackgroundConstrictor agonists set vascular tone through two coupling processes, one tied to (electromechanical), the other independent (pharmacomechanical) of membrane potential (VM). This arrangement raises an intriguing query: are they variably recruited such that each agonist elicits a range of vasomotor signatures, functionally biased towards one mechanism or the other? This query underlies this study and our examination of agonist-induced arterial constriction. MethodsMouse mesenteric arteries were exposed to a classic Gq/11 (phenylephrine) or Gq/11/G12/13 (U46619) coupled receptor agonist, and responses monitored in the absence and presence of L-type Ca2+ channel/protein kinase inhibitors. Contractile work was supplemented with measures of protein phosphorylation, VM, and cytosolic Ca2+; conceptual insights were enhanced with computational modeling. ResultsEach constrictor elicited a response curve that was attenuated and rightward shifted by nifedipine, findings aligned with functional bias; electromechanical coupling preceded pharmacomechanical, the latters importance rising with agonist concentration. Ensuing contractile and phosphorylation (CPI-17 & MYPT1 (T-855 & T-697)) measures revealed phenylephrine-induced pharmacomechanical coupling was tied to protein kinase C (PKC), while U46619 was tied to both PKC and Rho-kinase. A switch to pharmacomechanical coupling dominance occurred when agonist superfusion was replaced with discrete application to a small portion of artery. This switch was predicted by electromechanical modeling and supported by direct measures of VM and cytosolic Ca2+. ConclusionsOur work illustrates that constrictor agonists elicit functionally biased responses and that arteries toggle among contractile mechanisms, dependent on receptor signal bias, structural/electrical properties, and how agents are applied. We discuss how hemodynamic control is intimately tied to functional bias in both health and disease states, including but not limited to arterial vasospasm. O_FIG O_LINKSMALLFIG WIDTH=164 HEIGHT=200 SRC="FIGDIR/small/588016v1_ufig1.gif" ALT="Figure 1"> View larger version (62K): org.highwire.dtl.DTLVardef@11632b9org.highwire.dtl.DTLVardef@5c1144org.highwire.dtl.DTLVardef@719b08org.highwire.dtl.DTLVardef@795ec3_HPS_FORMAT_FIGEXP M_FIG C_FIG HighlightsO_LIAgonist-induced constrictor responses exhibit a "functional bias" toward electromechanical or pharmacomechanical coupling dependent on agent concentration and mode of application. C_LIO_LIElectromechanical coupling typically but not exclusively precedes pharmacomechanical, the latter rising to prominence with agent concentration. Pharmacomechanical coupling is mediated through receptor pathways linked to PKC and Rho-kinase, regulatory proteins that target the catalytic and targeting subunit of MLCP. C_LIO_LIFunctional bias is malleable and thus each agonist elicits a range of vasomotor signatures, each presumptively important in controlling blood flow delivery in space and time. C_LI