Contribution of α-adrenoceptor stimulation by phenylephrine to basal nitric oxide production in the isolated mouse aorta.

In the mouse aorta, contractions evoked by the α(1)-adrenoceptor agonist phenylephrine are strongly suppressed by the continuous production of nitric oxide (NO). We investigated whether phenylephrine itself stimulated NO production by activating endothelial α(2)-adrenoceptors. On a prostaglandin F(2α) contraction, the α(2)-adrenoceptor agonist 5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine (UK14304) induced 29.3 ± 7.4% relaxation, which was inhibited by 0.1 µM 2-[(4,5-Dihydro-1H-imidazol-2-yl)methyl]-2,3-dihydro-1-methyl-1H-isoindole (BRL44408) with a pKB' corresponding to α(2)-antagonism. In the presence of NO synthase blockers, UK14304 elicited small contractions above 1 µM that were inhibited by 0.1 µM prazosin, but not influenced by 0.1 µM rauwolscine. At 3 µM or higher concentrations, phenylephrine caused only modest relaxation (up to 7.4 ± 2.3%) of segments constricted with prostaglandin F(2α) in the presence of prazosin, which was abolished with 0.1 µM BRL44408. Furthermore, BRL44408 did not increase contractions induced with 1 µM phenylephrine. These results confirm that α(1)- but not α(2)-adrenoceptors are expressed on aortic smooth muscle cells, whereas endothelial cells only express α(2)-adrenoceptors. Moreover, phenylephrine exerted a very modest relaxing effect through nonspecific stimulation of α(2)-adrenoceptors, but only at concentrations higher than 1 µM. It is concluded that the high basal output of NO in the isolated mouse aorta is not due to stimulation of α-adrenoceptors.

Contribution of transient and sustained calcium influx, and sensitization to depolarization-induced contractions of the intact mouse aorta.

BACKGROUND: Electrophysiological studies of L-type Ca2+ channels in isolated vascular smooth muscle cells revealed that depolarization of these cells evoked a transient and a time-independent Ca2+ current. The sustained, non-inactivating current occurred at voltages where voltage-dependent activation and inactivation overlapped (voltage window) and its contribution to basal tone or active tension in larger multicellular blood vessel preparations is unknown at present. This study investigated whether window Ca2+ influx affects isometric contraction of multicellular C57Bl6 mouse aortic segments. RESULTS: Intracellular Ca2+ (Cai2+, Fura-2), membrane potential and isometric force were measured in aortic segments, which were clamped at fixed membrane potentials by increasing extracellular K+ concentrations. K+ above 20 mM evoked biphasic contractions, which were not affected by inhibition of IP3- or Ca2+ induced Ca2+ release with 2-aminoethoxydiphenyl borate or ryanodine, respectively, ruling out the contribution of intracellular Ca2+ release. The fast force component paralleled Cai2+ increase, but the slow contraction coincided with Cai2+ decrease. In the absence of extracellular Ca2+, basal tension and Cai2+ declined, and depolarization failed to evoke Cai2+ signals or contraction. Subsequent re-introduction of external Ca2+ elicited only slow contractions, which were now matched by Cai2+ increase. After Cai2+ attained steady-state, isometric force kept increasing due to Ca2+- sensitization of the contractile elements. The slow force responses displayed a bell-shaped voltage-dependence, were suppressed by hyperpolarization with levcromakalim, and enhanced by an agonist of L-type Ca2+ channels (BAY K8644). CONCLUSION: The isometric response of mouse aortic segments to depolarization consists of a fast, transient contraction paralleled by a transient Ca2+ influx via Ca2+ channels which completely inactivate. Ca2+ channels, which did not completely inactivate during the depolarization, initiated a second, sustained phase of contraction, which was matched by a sustained non-inactivating window Ca2+ influx. Together with sensitization, this window L-type Ca2+ influx is a major determinant of basal and active tension of mouse aortic smooth muscle.

Phospholipid scramblase 1 is secreted by a lipid raft-dependent pathway and interacts with the extracellular matrix protein 1 in the dermal epidermal junction zone of human skin.

We examined the interaction of ECM1 (extracellular matrix protein 1) using yeast two-hybrid screening and identified the type II transmembrane protein, PLSCR1 (phospholipid scramblase 1), as a binding partner. This interaction was then confirmed by in vitro and in vivo co-immunoprecipitation experiments, and additional pull-down experiments with GST-tagged ECM1a fragments localized this interaction to occur within the tandem repeat region of ECM1a. Furthermore, immunohistochemical staining revealed a partial overlap of ECM1 and PLSCR1 in human skin at the basal epidermal cell layer. Moreover, in human skin equivalents, both proteins are expressed at the basal membrane in a dermal fibroblast-dependent manner. Next, immunogold electron microscopy of ultrathin human skin sections showed that ECM1 and PLSCR1 co-localize in the extracellular matrix, and using antibodies against ECM1 or PLSCR1 cross-linked to magnetic immunobeads, we were able to demonstrate PLSCR1-ECM1 interaction in human skin extracts. Furthermore, whereas ECM1 is secreted by the endoplasmic/Golgi-dependent pathway, PLSCR1 release from HaCaT keratinocytes occurs via a lipid raft-dependent mechanism, and is deposited in the extracellular matrix. In summary, we here demonstrate that PLSCR1 interacts with the tandem repeat region of ECM1a in the dermal epidermal junction zone of human skin and provide for the first time experimental evidence that PLSCR1 is secreted by an unconventional secretion pathway. These data suggest that PLSCR1 is a multifunctional protein that can function both inside and outside of the cell and together with ECM1 may play a regulatory role in human skin.

Selective loss of basal but not receptor-stimulated relaxation by endothelial nitric oxide synthase after isolation of the mouse aorta.

Bioavailability of nitric oxide (NO) is mostly studied in isolated blood vessels. We investigated changes in basal and receptor-stimulated endothelial NO synthase (eNOS) activity after isolation of wild-type and Marfan mouse aorta. Starting 1h after dissection, basal NO release was assessed at hourly intervals by its ability to suppress isometric contractions in aortic segments. Relaxation induced by acetylcholine or α(2)-adrenoceptor agonist 5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine (UK14304) was used to study stimulated NOS activity. One hour after dissection, phenylephrine- or prostaglandin F(2α)-induced force attained only 17 ± 4% or 31 ± 7% of maximum tension in the presence of N(Ω)-nitro-l-arginine-methylesther (l-NAME), and contractions increased to 63 ± 6% and 82 ± 11%, respectively at 5h. In contrast, acetylcholine or UK14304 relaxation curves changed minimally. l-NAME and eNOS-deficiency abolished basal NO production, unlike inhibitors of neuronal (N(Ω)-propyl-l-arginine) or inducible (1400W) NOS. Acetylcholine-induced relaxation was abolished by l-NAME, strongly suppressed by eNOS-deficiency and attenuated by N(Ω)-propyl-l-arginine. In a bioassay based on diethylamine NONOate concentration-response curves the suppression of contractile forces was interpolated into NO equivalents. This showed exponential decay of basal NO, which occurred three times faster in aortas from mice with Marfan syndrome, while acetylcholine-induced relaxation remained unaltered. Immunoblotting showed unchanged eNOS expression, or phosphorylation at Ser1177, Ser617 or Thr495 between 1h and 4h, but Akt phosphorylation declined gradually. The dramatic loss of basal NO release after tissue isolation shows that timing is crucial when studying NO responses. The preservation of receptor-induced relaxation implies differential regulation of basal and stimulated eNOS activity, and phosphoinositide-3-kinase/Akt signalling seems specifically associated with basal eNOS activity.