Higher Ca2+ -sensitivity of arterial contraction in 1-week-old rats is due to a greater Rho-kinase activity.

AIM: During early post-natal development, arterial contraction depends less on Ca2+ -signalling pathways but more on changes in Ca2+ -sensitivity compared to adult animals. Whether this difference is related to Rho-kinase, one of the major players affecting Ca2+ -sensitivity, is unknown for intact vessels. Thus, we tested the hypothesis that Rho-kinase critically contributes to the higher Ca2+ -sensitivity of contraction in intact arteries of 1-week-old rats. METHODS: We studied 1-week-old, 4- to 5-week-old and 10- to 12-week-old rats performing isometric myography, Ca2+ -fluorimetry and Western blotting using intact saphenous arteries and arterial pressure measurements under urethane anaesthesia. RESULTS: In 10- to 12-week-old rats, methoxamine (MX) produced vasoconstriction associated with an increase in [Ca2+ ]i and Ca2+ -sensitivity. In contrast, in 1-week-old rats these contractions were accompanied only by an increase in Ca2+ -sensitivity. All MX-induced effects were reduced by the Rho-kinase inhibitor Y-27632; this reduction was complete only in 1-week-old rats. The Rho-kinase specific site Thr855 on MYPT1 was increasingly phosphorylated by MX in vessels of 1-week-old, but not 10- to 12-week-old rats; this effect was also inhibited completely by Y-27632. The Rho-kinase inhibitor fasudil in a dose not affecting the pressor response to MX in 4- to 5-week-old rats reduced it considerably in 1-week-old rats. CONCLUSION: Our results suggest that the higher Ca2+ -sensitivity of arterial contraction in 1-week-old compared to 10- to 12-week-old rats is due to a greater Rho-kinase activity. Constitutively active Rho-kinase contributes to MX-induced contraction in 10- to 12-week-old rats. In 1-week-old rats, additional Rho-kinase activation is involved. This remodelling of the Rho-kinase pathway is associated with its increased contribution to adrenergic arterial pressure responses.

Trophic action of sympathetic nerves reduces arterial smooth muscle Ca(2+) sensitivity during early post-natal development in rats.

AIM: A decrease in the Ca(2+) sensitivity of smooth muscle contraction is a hallmark of functional remodelling of blood vessels during development. However, the responsible factors are largely unknown. Here, we tested the hypothesis that the post-natal decline of arterial Ca(2+) sensitivity is the result of trophic effects of sympathetic nerves. METHODS: Contractile responses, intracellular Ca(2+) levels and protein expression profiles were compared in saphenous arteries from young (1- and 2-week-old) and adult rats using wire myography, Ca(2+) fluorimetry and Western blotting respectively. RESULTS: We observed a lower Ca(2+) sensitivity of contractions induced by methoxamine, an agonist of α1 -adrenoceptors, and U46619, an agonist of thromboxane A2 receptors, in arteries from adult as compared to young animals. Post-natal maturation was associated with stronger expression of regulatory proteins mediating Ca(2+) -dependent contraction (myosin light chain kinase (MLCK), myosin targeting subunit (MYPT1) and h-caldesmon) and weaker expression of proteins regulating Ca(2+) -independent contraction (Rho kinase, extracellular-regulated kinases (ERK1/2) and mitogen-activated protein kinases p38 MAPK) in vessels from adult rats. To eliminate the trophic action of sympathetic nerves, we performed lumbar sympathectomy in adult rats. This resulted in higher Ca(2+) sensitivity of agonist-induced contractions in denervated as compared to control arteries. Furthermore, denervated arteries contained less MLCK, MYPT1 and h-caldesmon and more ERK1/2 and p38 MAPK. CONCLUSIONS: Sympathetic denervation reverses developmental changes both in Ca(2+) sensitivity and in the expression of regulatory proteins back to the early post-natal phenotype in the rat saphenous artery. We conclude that trophic effects of sympathetic nerves govern functional remodelling of arteries during early post-natal development.