Dynamics of Ca2 handling in vascular smooth muscle

Intracellular Ca²⁺ is a key regulator of vascular contractility, and thereby of blood perfusion and pressure. Ca²⁺ signals are essential also for cell migration, proliferation and regulation of numerous enzymes. Besides producing vascular contraction smooth muscle cells may also modulate to a synthetic phenotype and proliferate as an initial stage in the atherosclerotic process. In this work we show that altered Ca²⁺ handling is an early step in phenotypic modulation, which might be important for promoting or regulating the process. Using tissue culture of rat tail and basilar arteries, large up-regulation of intracellular Ca²⁺ stores and of 'store operated' Ca²⁺ influx was evident, before changes in differentiation markers for the contractile state occurred. Further increase in intracellular Ca²⁺ storage could be induced by increasing the external load of Ca²⁺ during the culture period. The diverse roles of Ca²⁺ in signalling require mechanisms for selectivity, which appear to involve spatial and temporal coding. Vascular smooth muscle exhibits intracellular Ca²⁺ 'waves' and 'sparks'. Using culture in the presence of ryanodine, we achieved preparations lacking Ca²⁺ release from intracellular stores through the ryanodine receptor, but with intact inositol 1,4,5-trisphosphate sensitive release. These vessels lacked Ca²⁺ sparks but showed normal wave activity, indicating selective roles of the two modes of Ca²⁺ release for these different kinds of signals. Inhibition of metabolism caused an increase in wave frequency and a reduction in amplitude, with no effect on averaged tissue [Ca²⁺]i, or myosin phosphorylation. This suggests that alteration of the wave pattern might regulate contractile force produced by summation of asynchronous phasic contractions of individual cells. Many receptors, enzymes and signalling molecules are concentrated in cholesterol-rich membrane regions (caveolae), which thus may be important for spatial coding. Extraction of cholesterol from caveolae had little effect on wave generation, but disrupted the coupling from many cell membrane receptors to contraction.