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

Abstract

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 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.

Chronic hypoxia and VEGF differentially modulate abundance and organization of myosin heavy chain isoforms in fetal and adult ovine arteries

“…This disconnect between the effects of hypoxia on K ϩ -induced and stretch-dependent (myogenic) contractions demonstrates that hypoxic acclimatization can differentially influence select components of the contractile apparatus and may preferentially affect mechanisms coupling stretch to contraction. Candidate mechanisms potentially involved in this selective effect of hypoxia include calcium channels involved in mechanotransduction (27) and contractile proteins involved in myogenic regulation of myofilament calcium sensitivity (14,52). Interestingly, many of these mechanisms are strongly influenced by phenotypic transformation of smooth muscle (14,29,65).…”

“…Another possibility is that the spatial organization of MHC relative to SM-␣A is altered as a consequence of structural responses to hypoxic acclimatization. To test this hypothesis, our experimental approach included measurements of the fraction of SM-␣A colocalized with both of the main MHC isoforms using confocal microscopy as previously described (14). Colocalization of NM-MHC with SM-␣A was significantly decreased by hypoxia in fetal arteries but was unchanged in adult arteries (Fig.…”

“…In turn, intracellular organization and smooth muscle phenotype were defined by the patterns of colocalization of nonmuscle and smooth muscle myosin heavy chain (NM-MHC and SM-MHC) with SM-␣A (49,50). The experimental design focused first on the effects of chronic hypoxia on artery structure, contractile protein organization, and function using established methods for immunoblotting, confocal microscopy, and measurements of in vitro contractility (11,14). The same endpoints also helped define the structural and functional effects of organ culture with VEGF in a second tier of experiments.…”

Trophic action of sympathetic nerves reduces arterial smooth muscle Ca2+sensitivity during early post-natal development in rats

“…This is associated with accumulation of contractile proteins (Owens et al 2004, Payne et al 2006, Hutanu et al 2007, altered expression of membrane receptors (Goyal et al 2010) and changes in mechanisms that regulate the cytoplasmic Ca 2+ concentration ([Ca 2+ ] i ) (Gollasch et al 1998, Long et al 1999, Charles et al 2010. Early post-natal maturation is also marked by a gradual decrease in myofilament Ca 2+ sensitivity (Akopov et al 1997, Sandoval et al 2007 determined as the relationship between the level of contraction and the intracellular Ca 2+ concentration ([Ca 2+ ] i ) (Hirano 2007).…”

Hydrogen sulfide and cerebral microvascular tone in newborn pigs

“…The pathophysiological mechanisms of neonatal ischemic brain injury can be distinct from those of the adult and include different circulatory control mechanisms in a system not fully developed (8,11). Newborn animals are used to study mechanisms involved in regulation of blood flow to and within the newborn brain because, while there can be commonalities, mechanisms involved in regulation of cerebrovascular circulation are often different in newborn pigs and presumably newborn infants from those in older pigs, adult rodents, and adult humans (3,9,15,41,48,49,55). The present experiments measure piglet pial arterioles that are critical for control of cerebral blood flow (5,6,14,16,17).…”