Comparative study of calcium ion dynamics and contractile response in rat middle cerebral and basilar arteries

Endothelial TrkB receptor activation controls vascular tone of rat middle cerebral artery

Little is known on the cerebrovascular BDNF (brain-derived neurotrophic factor)/TrkB (tropomyosin related kinase B) pathway. This study investigated the contribution of endogenous endothelial BDNF to the control of vascular tone of rat middle cerebral artery (MCA) and the capacity of exogenous agonist of TrkB receptors to induce their relaxation. Endothelial cells constitutively expressed both BDNF and activated TrkB receptors. Supporting endothelial BDNF as an autocrine regulator of basal myogenic tone, incubation of MCA with the TrkB antagonist cyclotraxin B induced contraction as observed with incubation in the presence of inhibitors of nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF) production. Exposure of MCA with the TrkB agonist LM22A-4 that increased expression of TrkB receptors phosphorylated at tyrosine 816 induced relaxation of preconstricted MCA (EC₅₀ 6.7 × 10^-8 mol/L) as efficiently than acetylcholine (EC₅₀ 5.3 × 10^-8 mol/L). Finally, endothelium removal, exposure to a TrkB antagonist or to inhibitors of NO and EDHF production prevented the relaxant effect of LM22A-4. In conclusion, our study identified endothelial BDNF as a new autocrine regulator of vascular tone of MCA, thus making the endothelial BDNF/TrkB pathway an attractive target for strategies aiming to improve blood supply to the brain.

Molecular identification of P2X receptors in vascular smooth muscle cells from rat anterior, posterior, and basilar arteries

BACKGROUND

Purinergic P2X receptors in vascular smooth muscle cells (VSMCs) play an important role in physiological stimulatory responses to the extracellularly released ATP. The aim of this work was to identify molecular P2X receptor subunits in VSMCs isolated from rat anterior, posterior and basilar arteries using a number of contemporary laboratory techniques.

METHODS

P2X mediated ionic currents were recorded using amphotericin B perforated patch clamp method. Gene expression analysis was performed using RT-PCR in manually collected VSMCs. The expression of proteins was confirmed by fluorescent immunocytochemistry.

RESULTS

Under voltage clamp conditions VSMCs stimulated by application of 10 μmol/l selective P2X receptor agonist αβ-meATP, the biphasic currents consisting of rapidly rising rapidly desensitizing and slowly desensitizing components were observed in freshly isolated myocytes from all three arteries. Using RT-PCR, the expression of genes encoding only P2X1 and P2X4 receptor subunits was detected in preparations from all three arteries. The expression of corresponding P2X1 and P2X4 receptor subunit proteins was confirmed in isolated VSMCs.

CONCLUSIONS

Our work therefore identified that in major arteries of rat cerebral circulation VSMCs express only P2X1 and P2X4 receptors subunits. We can propose that these P2X receptor subunits participate in functional P2X receptor structures mediating ATP-evoked stimulatory responses in cerebral vascular myocytes in vivo.

Roles of cytosolic Ca2+ concentration and myofilament Ca2+ 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 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.