Phosphorylation of the 20,000-dalton light chain of myosin is closely correlated with cross-bridge cycling in arterial smooth muscle. Evidence is presented that dephosphorylation can produce an attached, noncycling cross-bridge (latch-bridge) which is responsible for the high economy of force maintenance in this tissue.

No abstract

Ascorbate reduces the oxidation rate of catecholamines and, by an independent mechanism, enhances rabbit aortic ring contractions initiated by catecholamines. The largest significantly different fractional increases in force produced by ascorbate enhancement of norepinephrine (NE), epinephrine, phenylpropanolamine (PPA), and ephedrine (Eph) are 5.5, 1.8, 1.6, and 1.3 times, respectively. In physiological salt solutions bubbled with 95% O(2) at 37 degrees C, NE, PPA, and Eph have oxidation rate constants of 1.24, 247, and 643 h, respectively. Ascorbate significantly enhances 100 nM NE contractions by at least twofold at all ascorbate concentrations >15 microM, including the entire physiological range of 40-100 microM. Ascorbate preloading and washout followed by NE exposure produces significantly greater contractions than NE without ascorbate preloading but significantly lower than NE simultaneously with ascorbate. Ascorbate does not enhance K(+)- or angiotensin II-induced contractions. Ascorbate enhancement of catecholamine contractions occurs in addition to the reduction in oxidation rate, because the increases in force occur faster than oxidation can occur, the increases occur with compounds that have negligible oxidation rates, and the increases occur when ascorbate and NE are not physically present together. These results are consistent with ascorbate acting on the adrenergic receptor. Ascorbate may play a role in shock and asthma treatments and potentiate the cardiovascular health consequences of PPA and Eph (Ephedra).

Ascorbate has previously been shown to enhance both alpha(1)- and beta(2)-adrenergic activity. This activity is mediated by ascorbate binding to the extracellular domain of the adrenergic receptor, which also decreases the oxidation rate of ascorbate. H1 histamine receptors have extracellular agonist or ascorbate binding sites with strong similarities to alpha(1-) and beta(2)-adrenergic receptors. Physiological concentrations of ascorbate (50 microM) significantly enhanced histamine contractions of rabbit aorta on the lower half of the histamine dose-response curve, increasing contractions of 0.1, 0.2, and 0.3 microM histamine by two- to threefold. Increases in ascorbate concentration significantly enhanced 0.2 microM histamine (5-500 microM ascorbate) and 0.3 microM histamine (15-500 microM ascorbate) in a dose-dependent manner. Histamine does not measurably oxidize over 20 h in oxygenated PSS at 37 degrees C. Thus the ascorbate enhancement is independent of ascorbate's antioxidant effects. Ascorbate in solution oxidizes rapidly. Transfected histamine receptor membrane suspension with protein concentration at >3.1 microg/ml (56 nM maximum histamine receptor) decreases the oxidation rate of 392 microM ascorbate, and virtually no ascorbate oxidation occurs at >0.0004 mol histamine receptor/mol ascorbate. Histamine receptor membrane had an initial ascorbate oxidation inhibition rate of 0.094 min.microg protein(-1).ml(-1), compared with rates for transfected ANG II membrane (0.055 min.microg protein(-1).ml(-1)), untransfected membrane (0.052 min.microg protein(-1).ml(-1)), creatine kinase (0.0082 min.microg protein(-1).ml(-1)), keyhole limpet hemocyanin (0.00092 min.microg protein(-1).ml(-1)), and osmotically lysed aortic rings (0.00057 min.microg wet weight(-1).ml(-1)). Ascorbate enhancement of seven-transmembrane-spanning membrane receptor activity occurs in both adrenergic and histaminergic receptors. These receptors may play a significant role in maintaining extracellular ascorbate in a reduced state.

The isotonic shortening velocity of swine carotid media tissues contracting in response to high K+, histamine, norepinephrine, or AC electrical stimulation rapidly increased to a maximum value and then declined to a steady-state level while force was still increasing or steady. The maximum shortening velocity calculated for no external load on the tissue (Vo) also decreased during the course of contractions when active stress remained constant. The fall in velocity with time was not Ca2+ dependent, because reductions in the [Ca2+] in high K+ solutions that significantly reduced the maximum stress (Fo) had no effect on Vo in the steady state. On washout of high K+ solutions, the ability of the tissue to shorten on isotonic quick release fell rapidly to low levels before isometric stress exhibited significant declines. The data indicate that cross-bridge cycling rates are modulated in this tissue. We suggest that this reflects the formation of attached, noncycling cross bridges (termed latch bridges), which constitute an internal load on the contractile system during tonic contractions.