Robert H. Cox

Differential expression of voltage-gated K(+) channel genes in arteries from spontaneously hypertensive and Wistar-Kyoto rats.

Abstract—Voltage-gated K+ currents play an important role in determining membrane potential, intracellular Ca2+, and contraction in arterial smooth muscle. In this study, the expression of genes encoding voltage-gated K+ channels of the Kv1.X family was compared in arteries from spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). Expression of Kv1.X in thoracic aorta, mesenteric arteries, tail artery, and heart was determined, both qualitatively and quantitatively, by reverse transcription–polymerase chain reaction. Our results demonstrate distinct but overlapping patterns of expression in vascular tissues. In general, Kv1.2 and Kv1.5 were most highly represented, and the levels of Kv1.2 were significantly larger in all tissues from SHR. Levels of Kv1.5 in arteries did not differ significantly between strains but were greater in SHR heart. Moderate levels of Kv1.3 and Kv&bgr;1.1 expression were also found in all tissues and were larger in SHR. Kv1.1 expression was not different between the 2 strains, and no significant expression of Kv1.4 (except in heart and aorta), Kv1.6, or Kv&bgr;2.1 was observed in either strain. Kv1.2 and Kv1.5 transcripts represent ≈1 to 2 parts/105 of total mesenteric arterial RNA with ≈2- to 5-fold lower levels in aorta and tail artery. Whole-cell voltage-gated K+ channel currents, recorded from mesenteric arterial myocytes, were larger in SHR than WKY (eg, at 0 mV: 7.3±0.8 versus 10.9±1.2 pA/pF). The voltage dependence of activation was more negative in SHR (V0.5: −20±4 mV versus −32±3 mV) but that of availability was not different. These results indicate that Kv1.X genes are differentially expressed between WKY and SHR (especially Kv1.2 and Kv&bgr;1.1). These differences in gene expression are associated with a greater voltage-gated K+ channel current density in SHR and shifted voltage-dependent activation compared with WKY. These differences may be a compensatory mechanism related to the membrane potential depolarization in SHR or some manifestation thereof.

Comparison of linearized wave propagation models for arterial blood flow analysis

Abstract Linearized wave propagation models which can be used to predict both transmission characteristics and hydraulic fluid impedance values based upon system input parameters are reviewed. Most of these models considered herein are concerned with the propagation of small, axisymmetric, harmonic disturbances through an incompressible, Newtonian fluid contained within a distensible tube of long length. The equations of motion of the fluid are usually described by the linearized Navier-Stokes equations. The equations of motion of the wall used in the various models showed a great deal of variability. According to the tube equations of motion and the boundary conditions employed, these models can be divided into three categories: constrained, thin-walled tube models; freely moving, thin-walled models; and freely moving, thick-walled models. The differences in the results predicted by these three groups are generally a function of the values of the system parameters. A comparison of the phase velocity and transmission per wavelength predicted by the different models is made. The differences in the parameter variations of the fluid impedance, resistance and inertance among the different models are also discussed. The comparisons demonstrate the existence of significant differences in the values of propagation constant and fluid impedance predicted by the different models.

Effects of Age on Ca2+ Currents in Small Mesenteric Artery Myocytes From Wistar-Kyoto and Spontaneously Hypertensive Rats

The purpose of this study was to test the hypothesis that differences in voltage-gated Ca2+ channels increase with age during the development of sustained hypertension in the spontaneously hypertensive rat (SHR). Using patch-clamp methods, we measured whole-cell Ca2+ currents in freshly isolated myocytes from small mesenteric arteries of juvenile (5 to 7 weeks), young (10 to 12 weeks), and mature (19 to 23 weeks) Wistar-Kyoto rats (WKY) and SHR. Indirect tail artery systolic pressure increased progressively with age in SHR (from 125 +/- 5 to 159 +/- 5 to 192 +/- 5 mm Hg) but only in the younger WKY (from 107 +/- 6 to 130 +/- 4 to 136 +/- 4 mm Hg). Peak Ca2+ current density (current per cell capacitance) was larger in SHR than WKY myocytes at all ages (at 6 weeks, 3.5 +/- 0.4 versus 2.3 +/- 0.2 pA/pF; at 12 weeks, 3.8 +/- 0.2 versus 3.1 +/- 0.2; at 20 weeks. 4.9 +/- 0.4 versus 3.3 +/- 0.4). Cell capacitance (surface area) was significantly smaller in 12-week-old SHR than WKY (25.2 +/- 1.1 versus 31.8 +/- 1.6 pF), but no differences were found in the 6- or 20-week-old groups. There were significant differences in Ca2+ current with strain, age, and voltage but no significant age-strain interactions. The ratio of peak Ca2+ current for SHR to that of WKY declined linearly with voltage at all ages suggesting differences in the voltage dependence of Ca2+ current activation. The voltage dependence of Ca2+ current was shifted to the left in SHR compared with WKY at all ages. Activation curves were shifted significantly in the negative-voltage direction only in 20-week-old SHR myocytes. We have found differences with age in Ca2+ current density and its voltage dependence in SHR compared with WKY during the phase of development in which blood pressure becomes established in the SHR. The net effect of these differences predicts a larger Ca2+ current in SHR at voltages in the physiological range of membrane potential.

Augmented Calcium Currents in Mesenteric Artery Branches of the Spontaneously Hypertensive Rat

The greater efficacy of organic channel blockers in lowering peripheral resistance and blood pressure in hypertensive subjects has been suggested to be the result of augmented calcium influx through L-type calcium channels in arterial smooth muscle. These studies were performed to determine whether differences exist in voltage-gated calcium channels of mesenteric artery branches from 20-week-old spontaneously hypertensive rats (SHR) compared with Wistar-Kyoto rats (WKY). Single myocytes were acutely isolated by collagenase and elastase treatment and studied at room temperature (approximately 20 degrees C) with the use of whole-cell, patch-clamp methods. Maximum values of calcium current measured at 0 mV from a holding potential of -90 mV were larger in SHR myocytes (105 +/- 11 versus 149 +/- 15 pA). Values of cell capacitance were smaller in SHR (29.5 +/- 1.3 pF) compared with WKY (35.0 +/- 1.5 pF) myocytes. Cell capacitance measures surface membrane area and, when used to normalize calcium currents, magnified the difference between WKY and SHR to approximately 47%. There was a larger percent reduction of maximum calcium current at holding potentials of -60 and -40 mV in SHR compared with WKY myocytes: for example, at -40 mV calcium current was reduced from values at -90 mV by -73 +/- 2% in SHR compared with -58 +/- 1% in WKY. When divided by the maximum current for each holding potential, the voltage dependence of normalized calcium currents for the two groups was completely superimposed. Difference currents were calculated by subtracting currents measured from holding potentials of -90 and -40 mV. The voltage dependence of difference currents was identical to that of the calcium currents measured from the two values of holding potential.(ABSTRACT TRUNCATED AT 250 WORDS)

Differences in K+ current components in mesenteric artery myocytes from WKY and SHR

Previous studies have documented increased K+ permeability of arterial smooth muscle in hypertension and suggested a role in altered arterial contractile function. To characterize the mechanisms responsible for these alterations, we determined the contribution of K+ current (IK) components to whole cell IK in freshly dispersed myocytes and tetraethylammonium (TEA)-induced contractile responses in mesenteric arteries of Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). Tetraethylammonium produced a larger tonic contractile response in SHR with a lower threshold compared to WKY (ie, 0.1 v 1 mmol/L), which was due in part to the larger Ca2+ current in SHR. Whole cell IK recorded by perforated patch methods was similar at a holding potential (HP) of -60 mV (IK60), but were larger in SHR when recorded from a HP of -20 mV (IK20). The selective blocker iberiotoxin (IbTX) was used to separate the contribution of voltage- (Kv) and calcium-dependent (KCa) components of IK60. The IK60 and IK20 component inhibited by 100 nmol/L IbTX (ie, KCa) was larger in SHR than in WKY myocytes, whereas the IbTX-insensitive IK60 component (ie, Kv) was larger in WKY. In the presence of IbTX, 1 and 10 mmol/L TEA inhibited a larger fraction of IK60 in SHR myocytes compared with WKY. The activation properties of the TEA-sensitive and TEA-insensitive Kv components determined by fitting a Boltzmann activation function to the current-voltage data, exhibited both group and treatment differences in the half maximal activation voltage (V0.5). The V0.5 of the TEA-sensitive Kv component was more positive than that of the TEA-insensitive component in both groups, and values for the V0.5 of both TEA-sensitive and TEA-insensitive components were more negative in SHR than WKY. These results show that SHR myocytes have larger KCa and smaller Kv current components compared with WKY. Furthermore, SHR myocytes have a larger TEA-sensitive Kv component. These differences may contribute to the differences in TEA contractions, resting membrane potential, Ca2+ influx, and KCa current reported in hypertensive arteries.

Determination of the True Phase Velocity of Arterial Pressure Waves in Vivo

Results of simultaneously recorded pressure, diameter, flow, and differential pressure from an arterial segment were studied by Fourier analysis. The method was applied to pressure propagation in the femoral artery of anesthetized dogs. Experimental values of phase velocity compared favorably with values predicted from a theoretical wave propagation model. The phase velocity was generally constant for frequencies over 35 rad/sec. At frequencies below 20 rad/sec, it decreased rapidly with decreasing frequency. The frequency variations of the true and apparent phase velocities were significantly different. The characteristic impedance of the femoral artery was a weak function of frequency arid nearly independent of the mean arterial pressure. High frequency values of the local fluid impedance above 50 rad/sec were significantly lower than those of the characteristic impedance. It was concluded that the method can be used to obtain reliable values of phase velocity in the physiological frequency range from in vivo measurements.

Age-Related Changes in Arterial Wall Mechanics and Composition of NIA Fischer Rats

Segments of carotid and tail arteries and descending thoracic aorta were obtained from the NIA colony of Fischer rats at ages 3, 12, 24 and 30 months. Measurements of pressure and diameter were made on intact cylindrical segments under conditions of active (147 mM K+) and passive (Ca2+-free and 2 mM EGTA) smooth muscle. These data were used to compute active and passive mechanics. Contiguous segments were used for the analysis of connective tissue, water and electrolyte contents. Passive stiffness of the carotid and tail arteries increased monotonically with increasing age. Collagen content in the aorta and tail artery generally increased with age, while elastin content decreased in the aorta and carotid artery. The ratio of collagen to elastin increased at all sites with age. Maximum values of active stress response (force development) increased from 3 to 12 months for the carotid artery, but decreased with age (at 24 and 30 months compared to 3 and 12-months) for the tail artery. Changes in relative cell content were such that active cellular force development was the same at all ages for the carotid artery but was smaller at 24 and 30 months compared to the younger animals for the tail artery. Decreased cellular force development by arterial smooth muscle is not an anatomically uniform finding in this animal model.

Baroreceptor reflex control of arterial hemodynamics in the dog.

The regional differentiation of carotid sinus control of arterial pressure-flow relationships was studied in chloralose-anesthetized dogs. Simultaneous pressure-flow measurements were made in the ascending aorta, the celiac artery, the cranial mesenteric artery, the renal artery, and the femoral artery. The carotid sinuses were bilaterally isolated and perfused with pulsatile pressure. The open-loop reflex gain was not symmetrical about and was maximum at pressures below the closed-loop operating point pressure. Changes in both peripheral resistance and cardiac output contributed significantly to the open-loop gain, with the former predominating. Aortic impedance for frequencies above 3 Hz was at a minimum at the closed-loop operating point and increased for both higher and lower values of carotid sinus pressure. For the frequency range from 3 to 9 Hz, regional impedance in all of the beds varied inversely with carotid sinus pressure. The sensitivity of the various beds to changes in carotid sinus pressure around the operating point increased in the order celiac < mesenteric < renal < femoral. Following vagotomy, operating point values of regional resistance and sensitivity were significantly increased. This fact suggests that the aortic arch receptors exert a significant influence on regional vascular impedances at operating point pressures. The fraction of cardiac output in the celiac, mesenteric, and renal beds was nearly independent of carotid sinus pressure before and after vagotomy, but that in the femoral bed increased with carotid sinus pressure. These results demonstrate the nonuniform nature of carotid sinus and aortic arch baroreceptor control of regional blood flow.

Augmented Contributions of Voltage-Gated Ca2 Channels to Contractile Responses in Spontaneously Hypertensive Rat Mesenteric Arteries

The observation that organic Ca2+ channel blockers are more effective in lowering blood pressure and peripheral resistance in hypertensive compared to normotensive subjects suggests that there is a greater contribution from voltage-gated Ca2+ channels (CaL) to vascular force maintenance in hypertensive arteries. This study tests this hypothesis by comparing the effects of Bay k 8644 and nisoldipine on basal force development, contractile responses to norepinephrine and serotonin, and Ca2+ currents (ICa) in mesenteric artery (MA) from Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). MA rings were used to record isometric contractions at Lmax. Single cells were isolated by collagenase plus elastase for measurement of CaL properties by patch-clamp methods. Contractile responses to Bay k 8644 were larger and more sensitive in SHR than WKY, and were larger in endothelium-denuded compared to intact rings. In SHR, the addition of 10 nmol/L Bay k 8644 increased contractile sensitivity to norepinephrine (NE) and serotonin (5HT), and increased maximum response to 5HT. In WKY, 10 nmol/L Bay k 8644 produced a small increase in 5HT sensitivity with no effect on maximum response, and had no effect on NE responses. In the presence of 1 mumol/L nisoldipine, the maximum response and the sensitivity to both NE and 5HT were decreased in both WKY and SHR with the inhibitory effects of nisoldipine being larger in SHR than WKY. Peak ICa was larger in SHR, and current-voltage curves were shifted toward more negative voltages compared to WKY. Bay k 8644 increased ICa in both WKY and SHR myocytes with no apparent difference in the magnitude of its effect when expressed as a percent of control ICa. These results suggest that CaL contribute significantly to tonic force maintenance as well as to agonist responses in MA from both WKY and SHR, but with a much larger contribution in SHR. Differences in the sensitivity of CaL to Bay k 8644 were not responsible for the differences in contractile responses to this agonist.

Comparison of K+ channel properties in freshly isolated myocytes from thoracic aorta of WKY and SHR☆

Altered function of smooth muscle cell K+ channels have been reported in hypertension, but the contribution of various K+ channel types to these changes has not been completely determined. The purpose of this study was to compare the contribution of K+ channel types to whole cell K+ currents recorded from isolated thoracic aorta myocytes of 13 to 15 week old Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). Cells were isolated by collagenase and elastase digestion, and K+ currents recorded using whole cell voltage clamp methods at room temperature. Cells were superfused with a solution containing (in mmol/ L) 140 NaCl, 5 KCl, 2 CaCl2, 1 MgCl2, 10 HEPES, and 10 glucose. Pipettes were filled with a solution containing (in mmol/L) 120 KCl, 5 NaCl, 5 MgATP, 20 HEPES, and 10 BAPTA. The K+ currents (IK) recorded from a holding potential (HP) of -80 mV were smaller in the SHR compared to those in WKY (for example, at 20 mV: WKY = 6.1 +/- 0.6 pA/pF and SHR = 3.7 +/- 0.2 pA/pF). Values of cell capacitance were not different between the two groups (WKY = 25.2 +/- 3.2 pF and SHR = 26.6 +/- 1.9 pF). A component of IK inhibited by voltage (Kv) over the range from -80 to -20 mV was smaller in SHR. The voltage dependence of Kv availability and activation were not significantly different between the two groups. IK recorded from a HP = -20 mV (KCa) was not different between the two groups. Difference currents calculated from IK measured at HP of -80 and -20 mV (that is, Kv) were smaller in SHR as was the fraction of IK inhibited by 4-aminopyridine. These results suggest that under conditions of low intracellular [Ca2+] there are no differences in KCa currents, but the Kv currents are smaller in SHR. Inhibition of Kv by 4-aminopyridine (0.1 to 10 mmol/L) caused larger increases in basal tone in WKY aorta. These results suggest that Kv channels contribute to resting K+ conductance in both WKY and SHR aorta, but with a relatively larger contribution in the WKY.