The increase in intracellular Ca²⁺ is a key event in vascular smooth muscle (VSM) contraction. In addition, a number of intracellular signaling pathways provide for Ca²⁺-sensitisation of the contraction enabling smooth muscle to develop higher force at lower [Ca²⁺]_in. Earlier we found that Ca²⁺-sensitivity of vascular smooth muscle contraction in newborn and 2 week old rats is higher than in adults, but the molecular basis of this phenomenon has not been explored.

As we know, the force of contraction is determined by phosphorylation level of myosin light chain (MLC) that, in turn, is dependent on the balance of MLC-kinases and MLC-phosphatase activities. It is considered that Protein kinase C and Rho-kinase lead to the inhibition of MLC-phosphotase activity and thereby sensitize MLC phosphorylation to Ca²⁺. Mitogen-activated protein kinases (MAPK) are also assumed to take part in Ca²⁺-sensitisation of VSM contraction.

We investigated the role of two members of MAPK family (ERK1/2 and p38) in saphenous artery contraction (small resistance artery) of 2 week old and adult rats. For this purpose we studied the effects of ERK1/2 and p38 inhibition with U0126 and SB202190 respectively (10^-5 M for both) on isometric contractile response to methoxamine (selective a₁-adrenoceptor agonist). The effect of SB202190 was greater compared with U0126 and more prominent in 2 week olds than in adults. Together, the inhibitors decreased maximal force in 2 week olds by about 70% and in adults only by about 30%.

As a second step we compared the expression levels of ERK1/2 and p38 in different aged groups of rats. Artery segments were snap-frozen in liquid N₂ and then homogenized in SDS-contained buffer. Proteins were separated with SDS-PAGE electrophoresis and transferred to PVDF membranes. All samples were normalized to GAPDH content. Membranes were incubated with anti-MAPK antibodies and then visualized with enhanced chemiluminescence (ECL). Total MAPK content was significantly higher in 2 week olds than in adults (about 3-fold for ERK1/2 and 5-fold for p38).

So the greater effect of MAPK inhibitors on the contraction of 2 week old arteries can be explained by the higher MAPK content in young animals. We suggest that ERK1/2 and p38 pathways are more important for VSM contraction in young rats and may take part in Ca²⁺-sensitisation.