C. Subah Packer

Chronic hypoxia impairs pulmonary venous smooth muscle contraction

Chronic hypoxia increases total pulmonary vascular resistance and causes pulmonary hypertension. Although the effect of chronic hypoxia on pulmonary arterial tissue has been extensively studied, very little is known about the effects on the pulmonary vein. The purpose of the present investigation was to determine the effect of chronic hypoxia on pulmonary venous reactivity to several vasoactive agonists and on the venous response to acute hypoxia. Isolated pulmonary venous rings were taken from rats exposed to 2, 7, and 14 days of hypoxia (FIO2 = 0.1). A decrease in the response of the pulmonary vein to KCl was observed after 14 days of hypoxia. The reactivity (maximum active force produced) of the pulmonary vein in response to phenylephrine (PE) was reduced after 7 days of hypoxia. The response of the pulmonary vein to angiotensin II (AII) was more sensitive to the effects of chronic hypoxia since decreased reactivity to angiotensin II occurred after only 2 days of hypoxia. Prolonged hypoxia (14 days) had no further effect on the decreased reactivities to PE and AII. The sensitivities of pulmonary venous muscle to PE and AII were decreased (increased ED50 values) by 2 days of chronic hypoxia, but tended to return to control levels after 7 and 14 days of hypoxia. However, the contractile response of the pulmonary vein to acute hypoxia was not changed even after 14 days of chronic hypoxia. These results suggest that chronic hypoxia: (1) impairs pulmonary venous smooth muscle contractility; (2) reduces pulmonary venous reactivity and sensitivity to phenylephrine and angiotensin II; and (3) does not alter the pulmonary venous contractile response to acute hypoxia.

Arterial Muscle Myosin Heavy Chains and Light Chains in Spontaneous Hypertension

Increased maximum velocity of shortening (Vmax), increased shortening ability (delta Lmax) and decreased relaxation rate have been reported for arterial smooth muscle from 16- to 18-week-old spontaneously, hypertensive rats (SHR) compared with age-matched normotensive Wistar-Kyoto rats (WKY). Vmax is dependent on actomyosin ATPase activity, and this activity is in turn dependent on the level of phosphorylation of the 20-kDa myosin light chain (MLC20) normally a function of calcium concentration. In this article, methods are described and data are presented from studies addressing possible intracellular regulatory mechanisms that might lead to the altered contractility of the SHR arterial muscle. In one study, myofibrillar protein was extracted from 16- to 18-week-old SHR and WKY caudal arterial muscle. The Mg(2+)-activated ATPase activity was measured under conditions where the Ca2+ concentration was controlled. In another study, the amount of myosin present and relative proportions of the myosin heavy chain (MHC) isoforms were determined by quantitative SDS-PAGE using heavy molecular weight standards and bovine serum albumin as the standard for concentration. In a third study, MLC20 phosphorylation levels in electrically stimulated arterial muscle were determined by urea glycerol gel electrophoresis and Western blot analyses. The SHR (n = 6) myofibrillar ATPase liberated 0.011 +/- 0.003 mumol Pi/mg myosin/min, which was significantly more than the 0.006 +/- 0.001 mumol Pi/mg myosin/min liberated by the WKY (n = 4) myofibrillar ATPase (P < 0.05). Consistent with the increased ATPase activity, phosphorylation of MLC20 was increased by 2.8 times as much in the SHR compared with the WKY electrically stimulated arterial muscle. However, there was no difference in MHC isoform pattern in the SHR compared with the WKY arterial muscle in contrast to the findings of at least one other laboratory. This discrepancy is discussed. The data reviewed in this article lead to the conclusions that an increased actin-activated myosin ATPase activity and MLC20 phosphorylation are likely responsible for the increased velocity of shortening previously reported in SHR arterial muscle and the increased ATPase activity is not a function of an increased myosin content or of altered MHC isoform pattern in the SHR muscle.

Myosin Heavy Chain Isoform Patterns Do Not Correlate with Force-Velocity Relationships in Pulmonary Arterial Compared with Systemic Arterial Smooth Muscle

Velocity of shortening is dependent on the myosin heavy chain (MHC) isoform pattern in both skeletal and cardiac muscle (Pagani and Julian, 1984). Furthermore, it has been reported that a shift in MHC isoform ratio occurs with certain physiological or pathophysiological changes such as hypertrophy and/or hyperplasia of striated muscles (Litten et al., 1974). Such shifts in MHC isoform proportions accompany concomitant changes in shortening velocity and ATPase activity (Alpert et al., 1979; Alpert and Mulieri, 1980). At least two different MHC isoforms have been reported to exist in various different smooth muscles (Sparrow et al., 1987). The 200 kDa form and the 204 kDa form have been designated MHC1 and MHC2, respectively. The ratio of MHC1:MHC2 has been shown to vary dependent on smooth muscle type, animal species, stage of development, and under certain different physiological or pathophysiological conditions for the same muscle type (Sparrow et al., 1987; Mohammed and Sparrow, 1988). However, no functional correlation has yet been made between MHC isoform ratio and shortening velocity for smooth muscle. Therefore, the purpose of this study was to compare force-velocity (F-V) relationships and MHC isoform ratios from two different arterial muscles (pulmonary versus caudal) from the same species (rat).

Oxidized Low Density Lipoprotein (OX-LDL) Induced Arterial Muscle Contraction Signaling Mechanisms

Oxidized low-density lipoprotein cholesterol (OX-LDL), a reactive oxidant, forms when reactive oxygen spe- cies interact with LDL. Elevated OX-LDL may contribute to high blood pressure associated with diseases such as diabetes and obesity. The current study objective was to determine if OX-LDL is a vasoconstrictor acting through the OX-LDL re- ceptor (LOX1) on arterial smooth muscle and elucidate the intracellular signaling mechanism. Arteries were extracted from Sprague-Dawley rats (SD) and obese F1 offspring (ZS) of Zucker diabetic fatty rats (ZDF) x spontaneously hyper- tensive heart failure rats (SHHF). Pulmonary arterial and aortic rings and caudal arterial helical strips were attached to force transducers in muscle baths. Arterial preparations were contracted with high KCl to establish maximum force devel- opment in response to membrane depolarization (Po). Addition of OX-LDL caused contractions of varying strength de- pendent on the arterial type. OX-LDL contractions were normalized to % Po. Caudal artery was more reactive to OX-LDL than aorta or pulmonary artery. Interestingly, LOX1 density varied with arterial type in proportion to the magnitude of the contractile response to OX-LDL. OX-LDL contractions in the absence of calcium generated about 50% as much force as in normal calcium. Experiments with myosin light chain kinase and Rho kinase inhibitors, ML-9 and Y-27632, suggest OX-LDL induced contraction is mediated by additive effects of two distinct signaling pathways activated concomitantly in the presence of calcium. Results may impact development of new therapeutic agents to control hypertension associated with disorders in which circulating LDL levels are high in a high oxidizing environment.

Platelet Activating Factor Causes Relaxation of Isolated Pulmonary Artery and Aorta

Platelet activating factor (PAF) is a lipid chemical mediator produced by a variety of activated cells, including basophils, eosinophils, platelets, monocytes, endothelial cells, macrophages, and neutrophils (Tamura, 1987; Vercellotti, 1988). A number of pathophysiological responses, such as inflammatory and allergic reactions, bronchoconstriction, platelet aggregation, pulmonary hypertension, anaphylactic shock, endotoxic shock, and systemic hypotension are mediated by PAF (Benveniste and Vargaftig, 1983; Heffner, 1983a; 1983b; Terashita, 1985; 1987; Hanahan, 1986; Barnes, 1988). These pathological conditions are linked to changes in smooth muscle tone. For example, bronchoconstriction is due to the increase in tone of airway smooth muscle and hypotension is due to the decrease in tone of vascular smooth muscle.

Gender Differences in the Development of Pulmonary Hypertension (PH)

RESULTS MCT at the given dose is highly toxic to male rats when compared to female rats that yielded in high mortality (35% vs. 0.03%, p = 0.0001). Vascular endothelial damage caused by MCT triggers 12× more ET-l synthesis/secretion in male rats whereas only 1.3× increase ADM levels. In the case of female rats, ADM secretion is increased with the increase in ET1. Relaxation in response to ACh, ADM, and CGRP was endothelium-dependent. All three relaxing agents produced less relaxation of hypertensive pulmonary arterial rings. ADM and CGRP induced relaxation of NE contractions was 2.4× greater in female compared to male control vascular rings (p < 0.004) and 5.5× greater in female compared with male hypertensive rings (p < 0.005).

Norepinephrine Stimulates Inositol Trisphosphate Formation in Rat Pulmonary Arteries

Although the discovery of the “phosphoinositide effect” occurred over 35 years ago, its mechanisms were explored only over the last decade. It now is clear that hydrolysis of phosphoinositides generates second messengers for multiple cellular functions when receptors are activated by a wide array of hormones and agonists in a variety of cell types (for review see Rana and Hokin, 1990). Early studies focused on the role of phosphoinositides on regulation of secretion or control of release of secretory cell contents (Hokin and Hokin, 1953, 1960; Freinkel, 1957; Hokin et al., 1958, 1963; Axen et al., 1983). In recent years, studies of the physiological effects of phosphoinositide hydrolysis have extended to such cell types and tissues as cerebral cortical slices (Kendall and Nahorski, 1984), sympathetic ganglia (Bone et al., 1984), adrenal glomerulosa cells (Kojima et al., 1986), rod outer segments (Brown et al., 1987), epithelium (Anderson and Welsh, 1990), leukocytes (Bradford and Rubin, 1986), skeletal muscle (Volpe et al., 1985), and smooth muscle (Akhtar and Abdel-Latif, 1980; Bielkiewicz-vollrath et al., 1987).