Roles of cytosolic Ca²⁺ concentration and myofilament Ca ²⁺ sensitization in age-dependent cerebrovascular myogenic tone

In 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 Ca2+ handling differs markedly between contractile and noncontractile smooth muscle, the present study specifically tested the hypothesis that the relative contributions of Ca2+ 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 Ca2+ sensitivity measured in β-escin-permeabilized arteries increased with pressure in P14 but not adult MCAs. Cyclopiazonic acid (an inhibitor of Ca2+ release from the sarcoplasmic reticulum) increased diameter and reduced Ca2+ in adult MCAs but increased diameter with no apparent change in Ca2+ in P14 MCAs. La3+ (Ca 2+ influx inhibitor) increased diameter and decreased Ca2+ in adult MCAs, but in P14 MCAs, La3+ increased diameter with no apparent change in Ca2+. After treatment with both La3+ and CPA, diameters were passive in both adult and P14 MCAs, but Ca2+ 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 Ca2+ sensitization and Ca2+ influx to maintain myogenic reactivity. The inability of La3+ to reduce cytosolic Ca2+ in the pup MCA appears due to La3+-insensitive noncontractile smooth muscle cells, which contribute to the spatially averaged measurements of Ca2+ but not contraction. Copyright © 2010 the American Physiological Society.