Lack of uptake, release and action of UTP at sympathetic perivascular nerve terminals in rabbit ear artery

Type 1 inositol 1,4,5-trisphosphate receptors mediate UTP-induced cation currents, Ca2+ signals, and vasoconstriction in cerebral arteries

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) regulate diverse physiological functions, including contraction and proliferation. There are three IP(3)R isoforms, but their functional significance in arterial smooth muscle cells is unclear. Here, we investigated relative expression and physiological functions of IP(3)R isoforms in cerebral artery smooth muscle cells. We show that 2-aminoethoxydiphenyl borate and xestospongin C, membrane-permeant IP(3)R blockers, reduced Ca(2+) wave activation and global intracellular Ca(2+) ([Ca(2+)](i)) elevation stimulated by UTP, a phospholipase C-coupled purinergic receptor agonist. Quantitative PCR, Western blotting, and immunofluorescence indicated that all three IP(3)R isoforms were expressed in acutely isolated cerebral artery smooth muscle cells, with IP(3)R1 being the most abundant isoform at 82% of total IP(3)R message. IP(3)R1 knockdown with short hairpin RNA (shRNA) did not alter baseline Ca(2+) wave frequency and global [Ca(2+)](i) but abolished UTP-induced Ca(2+) wave activation and reduced the UTP-induced global [Ca(2+)](i) elevation by approximately 61%. Antibodies targeting IP(3)R1 and IP(3)R1 knockdown reduced UTP-induced nonselective cation current (I(cat)) activation. IP(3)R1 knockdown also reduced UTP-induced vasoconstriction in pressurized arteries with both intact and depleted sarcoplasmic reticulum (SR) Ca(2+) by approximately 45%. These data indicate that IP(3)R1 is the predominant IP(3)R isoform expressed in rat cerebral artery smooth muscle cells. IP(3)R1 stimulation contributes to UTP-induced I(cat) activation, Ca(2+) wave generation, global [Ca(2+)](i) elevation, and vasoconstriction. In addition, IP(3)R1 activation constricts cerebral arteries in the absence of SR Ca(2+) release by stimulating plasma membrane I(cat).

Calcium mobilization and spontaneous transient outward current characteristics upon agonist activation of P2Y2 receptors in smooth muscle cells

A quantitative model is provided that links the process of metabotropic receptor activation and sequestration to the generation of inositol 1,4,5-trisphosphate, the subsequent release of calcium from the central sarcoplasmic reticulum, and the consequent release of calcium from subsarcolemma sarcoplasmic reticulum that acts on large-conductance potassium channels to generate spontaneous transient outward currents (STOCs). This model is applied to the case of STOC generation in vascular A7r5 smooth muscle cells that have been transfected with a chimera of the P2Y(2) metabotropic receptor and green fluorescent protein (P2Y(2)-GFP) and exposed to the P2Y(2) receptor agonist uridine 5'-triphosphate. The extent of P2Y(2)-GFP sequestration from the membrane on exposure to uridine 5'-triphosphate, the ensuing changes in cytosolic calcium concentration, as well as the interval between STOCs that are subsequently generated, are used to determine parameter values in the model. With these values, the model gives a good quantitative prediction of the dynamic changes in STOC amplitude observed upon activation of metabotropic P2Y(2) receptors in the vascular smooth muscle cell line.

An endothelium-derived factor modulates purinergic neurotransmission to mesenteric arterial smooth muscle of hamster

The interaction between the endothelium and purinergic perivascular nerves was investigated by measuring the changes in amplitude of excitatory junction potential (EJP) of smooth muscle cells in hamster mesenteric arteries (100-350 microm). Uridin-5'-triphosphate (UTP) (100 microM) applied to endothelium-intact preparations evoked a hyperpolarization of 17.0 +/- 0.7 mV (n=46). During this hyperpolarization, the amplitude of electrically evoked EJPs was inhibited to about 50% of that of the control. In endothelium-denuded preparations, UTP (100 microM) neither hyperpolarized the smooth muscle nor inhibited the amplitude of the EJP. Neither a nitric oxide (NO) synthase inhibitor, Nomega-nitro-L-arginine methyl ester (L-NAME) (100 microM), nor a cyclooxygenase inhibitor, indomethacin (1 microM), had an effect on the UTP-evoked hyperpolarization and inhibition of the electrically evoked EJP. The UTP-evoked membrane hyperpolarization and inhibition of the EJP amplitude was antagonized by the P2Y receptor antagonist, cibacron blue (100 microM). Endothelium-derived hyperpolarizing factor (EDHF)-mediated hyperpolarization was inhibited by either adventitial or intimal application of apamin (0.1 micro and charybdotoxin (0.1 microM). However, the EJP inhibition was still present. In apamin- and charybdotoxin-treated preparations, focal application of adenosine 5'-triphosphate (ATP) (10 mM) evoked a depolarization of 15.5 +/- 1.3 mV (n=15). This postjunctional response was not modified by UTP (15.3 +/- 1.7 mV, n=4, P>0.05). These results suggest that exogenously applied UTP activates P2Y receptors of endothelium to release endothelium-derived factors, which in turn inhibit ATP release from purinergic nerves.