American journal of physiology. Heart and circulatory physiology

1522-153929942010OctAmerican journal of physiology. Heart and circulatory physiologyAm J Physiol Heart Circ PhysiolRoles of cytosolic Ca2+ concentration and myofilament Ca2+ sensitization in age-dependent cerebrovascular myogenic tone.H1034H1044H1034-4410.1152/ajpheart.00214.2010In light of evidence that immature arteries contain a higher proportion of noncontractile smooth muscle cells than found in fully differentiated mature arteries, the present study explored the hypothesis that age-related differences in the smooth muscle phenotype contribute to age-related differences in contractility. Because Ca(2+) handling differs markedly between contractile and noncontractile smooth muscle, the present study specifically tested the hypothesis that the relative contributions of Ca(2+) influx and myofilament sensitization to myogenic tone are upregulated, whereas Ca(2+) release is downregulated, in immature [14 days postnatal (P14)] compared with mature (6 mo old) rat middle cerebral arteries (MCAs). Myofilament Ca(2+) sensitivity measured in β-escin-permeabilized arteries increased with pressure in P14 but not adult MCAs. Cyclopiazonic acid (an inhibitor of Ca(2+) release from the sarcoplasmic reticulum) increased diameter and reduced Ca(2+) in adult MCAs but increased diameter with no apparent change in Ca(2+) in P14 MCAs. La(3+) (Ca(2+) influx inhibitor) increased diameter and decreased Ca(2+) in adult MCAs, but in P14 MCAs, La(3+) increased diameter with no apparent change in Ca(2+). After treatment with both La(3+) and CPA, diameters were passive in both adult and P14 MCAs, but Ca(2+) was decreased only in adult MCAs. To quantify the fraction of smooth muscle cells in the fully differentiated contractile phenotype, extents of colocalization between smooth muscle α-actin and SM2 myosin heavy chain were determined and found to be at least twofold greater in adult than pup MCAs. These data suggest that compared with adult MCAs, pup MCAs contain a greater proportion of noncontractile smooth muscle and, as a consequence, rely more on myofilament Ca(2+) sensitization and Ca(2+) influx to maintain myogenic reactivity. The inability of La(3+) to reduce cytosolic Ca(2+) in the pup MCA appears due to La(3+)-insensitive noncontractile smooth muscle cells, which contribute to the spatially averaged measurements of Ca(2+) but not contraction.CharlesShelton MSMCenter for Perinatal Biology, Division of Physiology, School of Medicine, Loma Linda University, Loma Linda, CA 92350, USA.ZhangLuboLCipollaMarilyn JMJBuchholzJohn NJNPearceWilliam JWJengHD-31266HDNICHD NIH HHSUnited StatesHL-64867HLNHLBI NIH HHSUnited StatesR01 HL083966HLNHLBI NIH HHSUnited StatesR01 HL089012HLNHLBI NIH HHSUnited StatesR01 HL054120HLNHLBI NIH HHSUnited StatesR13 HD044630HDNICHD NIH HHSUnited StatesHL-54120HLNHLBI NIH HHSUnited StatesJournal ArticleResearch Support, N.I.H., ExtramuralResearch Support, Non-U.S. Gov't20100716