Electrical properties of iridial arterioles of the rat

Vascular structural and functional changes: their association with causality in hypertension: models, remodeling and relevance

Essential hypertension is a complex multifactorial disease process that involves the interaction of multiple genes at various loci throughout the genome, and the influence of environmental factors such as diet and lifestyle, to ultimately determine long-term arterial pressure. These factors converge with physiological signaling pathways to regulate the set-point of long-term blood pressure. In hypertension, structural changes in arteries occur and show differences within and between vascular beds, between species, models and sexes. Such changes can also reflect the development of hypertension, and the levels of circulating humoral and vasoactive compounds. The role of perivascular adipose tissue in the modulation of vascular structure under various disease states such as hypertension, obesity and metabolic syndrome is an emerging area of research, and is likely to contribute to the heterogeneity described in this review. Diversity in structure and related function is the norm, with morphological changes being causative in some beds and states, and in others, a consequence of hypertension. Specific animal models of hypertension have advantages and limitations, each with factors influencing the relevance of the model to the human hypertensive state/s. However, understanding the fundamental properties of artery function and how these relate to signalling mechanisms in real (intact) tissues is key for translating isolated cell and model data to have an impact and relevance in human disease etiology. Indeed, the ultimate aim of developing new treatments to correct vascular dysfunction requires understanding and recognition of the limitations of the methodologies used.

Autoregulation and conduction of vasomotor responses in a mathematical model of the rat afferent arteriole

We have formulated a mathematical model for the rat afferent arteriole (AA). Our model consists of a series of arteriolar smooth muscle cells and endothelial cells, each of which represents ion transport, cell membrane potential, and gap junction coupling. Cellular contraction and wall mechanics are also represented for the smooth muscle cells. Blood flow through the AA lumen is described by Poiseuille flow. The AA model's representation of the myogenic response is based on the hypothesis that changes in hydrostatic pressure induce changes in the activity of nonselective cation channels. The resulting changes in membrane potential then affect calcium influx through changes in the activity of the voltage-gated calcium channels, so that vessel diameter decreases with increasing pressure values. With this configuration, the model AA maintains roughly stable renal blood flow within a physiologic range of blood flow pressure. Model simulation of vasoconstriction initiated from local stimulation also agrees well with findings in the experimental literature, notably those of Steinhausen et al. (Steinhausen M, Endlich K, Nobiling R, Rarekh N, Schütt F. J Physiol 505: 493-501, 1997), which indicated that conduction of vasoconstrictive response decays more rapidly in the upstream flow direction than downstream. The model can be incorporated into models of integrated renal hemodynamic regulation.

Non-linear relationship between hyperpolarisation and relaxation enables long distance propagation of vasodilatation

Blood flow is adjusted to tissue demand through rapidly ascending vasodilatations resulting from conduction of hyperpolarisation through vascular gap junctions. We investigated how these dilatations can spread without attenuation if mediated by an electrical signal. Cremaster muscle arterioles were studied in vivo by simultaneously measuring membrane potential and vessel diameter. Focal application of acetylcholine elicited hyperpolarisations which decayed passively with distance from the local site,while dilatation spread upstream without attenuation. Analysis of simultaneous recordings at the local site revealed that hyperpolarisation and dilatation were only linearly related over a restricted voltage range to a threshold potential, beyond which dilatation was maximal. Experimental data could be simulated in a computational model with electrotonic decay of hyperpolarisation but imposition of this threshold. The model was tested by reducing the amplitude of the local hyperpolarisation which led to entry into the linear range closer to the local site and decay of dilatation. Serial section electron microscopy and light dye treatment confirmed that the spread of dilatation occurred through the endothelium and that the two cell layers were tightly coupled. Generality of the mechanism was demonstrated by applying the model to the attenuated propagation of dilatation found in larger arteries.We conclude that long distance spread of locally initiated dilatations is not due to a regenerative electrical phenomenon, but rather a restricted linear relationship between voltage and vessel tone, which minimises the impact of electrotonic decay of voltage. Disease-related alterations in endothelial coupling or ion channel expression could therefore decrease the ability to adjust blood flow to meet metabolic demand.

Gap junctions in the control of vascular function

Direct intercellular communication via gap junctions is critical in the control and coordination of vascular function. In the cardiovascular system, gap junctions are made up of one or more of four connexin proteins: Cx37, Cx40, Cx43, and Cx45. The expression of more than one gap-junction protein in the vasculature is not redundant. Rather, vascular connexins work in concert, first during the development of the cardiovascular system, and then in integrating smooth muscle and endothelial cell function, and in coordinating cell function along the length of the vessel wall. In addition, connexin-based channels have emerged as an important signaling pathway in the astrocyte-mediated neurovascular coupling. Direct electrical communication between endothelial cells and vascular smooth muscle cells via gap junctions is thought to play a relevant role in the control of vasomotor tone, providing the signaling pathway known as endothelium-derived hyperpolarizing factor (EDHF). Consistent with the importance of gap junctions in the regulation of vasomotor tone and arterial blood pressure, the expression of connexins is altered in diseases associated with vascular complications. In this review, we discuss the participation of connexin-based channels in the control of vascular function in physiologic and pathologic conditions, with a special emphasis on hypertension and diabetes.

Dissection of two Cx37-independent conducted vasodilator mechanisms by deletion of Cx40: electrotonic versus regenerative conduction

Conduction of changes in diameter plays an important role in the coordination of peripheral vascular resistance and, thereby, in the control of arterial blood pressure. It is thought that conduction of vasomotor signals relies on the electrotonic spread of changes in membrane potential from a site of stimulation through gap junctions connecting the cells of the vessel wall. To explore this idea, we stimulated a short segment of mouse cremasteric arterioles with an application, via micropipette, of ACh, an endothelium-dependent vasodilator, or pinacidil, an ATP-sensitive K+ channel opener. Vasodilations were evaluated at the stimulation site (local) and at 500, 1,000, and 2,000 microm upstream. The vasodilator response evoked by direct arteriolar hyperpolarization induced by pinacidil decayed rapidly with distance, as expected for the passive spread of an electrical signal. Deletion of the gap junction proteins connexin37 or connexin40 did not alter the conduction of pinacidil-induced vasodilation. In contrast to pinacidil, the vasodilator response activated by ACh spread along the entire vessel without decrement. Although the ACh-induced conducted vasodilation was similar in wild-type and connexin37 knockout mice, deletion of connexin40 converted the nondecremental conducted response activated by ACh into one similar to that of pinacidil, with a decline in magnitude along the vessel length. These results suggest that ACh activates a mechanism of regenerative conduction of vasodilator responses. Connexin40 is essential for the ACh-activated regenerative vasodilator mechanism. However, neither connexin40 nor connexin37 is indispensable for the electrotonic spread of hyperpolarizing signals.

A-type potassium current in retinal arteriolar smooth muscle cells

PURPOSE

By their control of membrane potential and intracellular free Ca(2+) ([Ca(2+)](i)), K(+) currents are pivotal in the regulation of arterial smooth muscle tone. The goal of the present study was to identify and characterize the A-type K(+) current in retinal microvascular smooth muscle (MVSM) and to examine its role in modulating membrane potential and cellular contractility.

METHODS

Whole-cell perforated patch-clamp recordings were made from MVSM cells within intact isolated arteriolar segments. Before patch-clamping, retinal arterioles were anchored in the physiological recording bath and perfused with an enzyme cocktail to remove surface basal lamina and to uncouple electrically the endothelial cells from the overlying MVSM cells.

RESULTS

K(+) currents were activated by depolarizing steps from -80 to +100 mV in 20-mV increments. A dominant, noninactivating current was elicited by depolarization to potentials positive of -50 mV. Inhibition of this current by 100 nM of the Ca(2+)-activated K(+) channel blocker, Penitrem A, revealed a rapidly inactivating K(+) current that resembled an A-type current. The A-type current was insensitive to tetraethylammonium (TEA) at 1 mM, but was partially suppressed by higher concentrations (10 mM). 4-Aminopyridine (10 mM; 4-AP) completely blocked the A-type current. The 4-AP-sensitive transient current was activated at a potential of -60 mV with peak current densities averaging 29.7 +/- 5.68 pA/pF at +60 mV. The voltage of half-inactivation was -28.3 +/- 1.9 mV, and the time constant for recovery from inactivation at +60 mV was 118.7 +/- 7.9 ms. Under current-clamp conditions 4-AP depolarized the membrane potential by approximately 3 to 4 mV and triggered small contractions and relaxations of individual MVSM cells within the walls of the arterioles.

CONCLUSIONS

A-type current is the major voltage-dependent K(+) current in retinal MVSM and appears to play a physiological role in suppressing cell excitability and contractility.

Voltage independence of vasomotion in isolated irideal arterioles of the rat

The cellular mechanisms underlying vasomotion of irideal arterioles from juvenile rats have been studied using electrophysiological methods, ratiometric calcium measurements and video microscopy. Vasomotion was not affected by removal of the endothelium. Spontaneous contractions were preceded by spontaneous depolarizations. Both were abolished by the intracellular calcium chelator, BAPTA AM (20 microM), but not by ryanodine (10 microM), suggesting a dependence on the cyclical release of calcium from intracellular stores, other than those operated by ryanodine receptors. Oscillations were little changed when the membrane potential of short segments of arteriole was either depolarized or hyperpolarized. When the segments were voltage clamped, oscillating inward currents were recorded, indicating that the changes in membrane potential were voltage independent. Vasomotion was preceded by intracellular calcium oscillations and both were abolished by inhibitors of phospholipase C (U73122, 10 microM), phospholipase A(2) (AACOCF(3), 30 microM) and protein kinase C (chelerythrine chloride, 5 microM, and myristoylated protein kinase C peptide, 10 microM). Inhibition of vasomotion by the dual lipoxygenase and cyclo-oxygenase inhibitor, NDGA (10 microM), the lipoxygenase inhibitor, ETI (1 microM) but not by the cyclo-oxygenase inhibitors, aspirin (10 microM) and indomethacin (10 microM), or the cytochrome P450 inhibitor 17-ODYA (10 microM), suggested an involvement of the lipoxygenase pathway. The observations suggest that vasomotion of iris arterioles is voltage independent and results from the cyclical release of calcium from IP(3)-sensitive stores which are activated by cross talk between the phospholipase C and phospholipase A(2) pathways in vascular smooth muscle.

Nifedipine blocks Ca2+ store refilling through a pathway not involving L-type Ca2+ channels in rabbit arteriolar smooth muscle

This study assessed the contribution of L-type Ca2+ channels and other Ca2+ entry pathways to Ca2+ store refilling in choroidal arteriolar smooth muscle. Voltage-clamp recordings were made from enzymatically isolated choroidal microvascular smooth muscle cells and from cells within vessel fragments (containing < 10 cells) using the whole-cell perforated patch-clamp technique. Cell Ca2+ was estimated by fura-2 microfluorimetry. After Ca2+ store depletion with caffeine (10 mM), refilling was slower in cells held at -20 mV compared to -80 mV (refilling half-time was 38 +/- 10 and 20 +/- 6 s, respectively). To attempt faster refilling via L-type Ca2+ channels, depolarising steps from -60 to -20 mV were applied during a 30 s refilling period following caffeine depletion. Each step activated L-type Ca2+ currents and [Ca2+]i transients, but failed to accelerate refilling. At -80 mV and in 20 mM TEA, prolonged caffeine exposure produced a transient Ca2+-activated Cl- current (I(Cl)(Ca)) followed by a smaller sustained current. The sustained current was resistant to anthracene-9-carboxylic acid (1 mM; an I(Cl)(Ca) blocker) and to BAPTA AM, but was abolished by 1 microM nifedipine. This nifedipine-sensitive current reversed at +29 +/- 2 mV, which shifted to +7 +/- 5 mV in Ca2+-free solution. Cyclopiazonic acid (20 microM; an inhibitor of sarcoplasmic reticulum Ca2+-ATPase) also activated the nifedipine-sensitive sustained current. At -80 mV, a 5 s caffeine exposure emptied Ca2+ stores and elicited a transient I(Cl)(Ca). After 80 s refilling, another caffeine challenge produced a similar inward current. Nifedipine (1 microM) during refilling reduced the caffeine-activated I(Cl)(Ca) by 38 +/- 5 %. The effect was concentration dependent (1-3000 nM, EC50 64 nM). In Ca2+-free solution, store refilling was similarly depressed (by 46 +/- 6 %). Endothelin-1 (10 nM) applied at -80 mV increased [Ca2+]i, which subsided to a sustained 198 +/- 28 nM above basal. Cell Ca2+ was then lowered by 1 microM nifedipine (to 135 +/- 22 nM), which reversed on washout. These results show that L-type Ca2+ channels fail to contribute to Ca2+ store refilling in choroidal arteriolar smooth muscle. Instead, they refill via a novel non-selective store-operated cation conductance that is blocked by nifedipine.

Sodium-potassium-ATPase electrogenicity in cerebral precapillary arterioles

Electrogenicity of the Na(+)/K(+) pump has the capability to generate a large negative membrane potential independently of ion-channel current. The high background membrane resistance of arterioles may make them susceptible to such an effect. Pump current was detected by patch-clamp recording from smooth muscle cells in fragments of arterioles (diameter 24-58 microm) isolated from pial membrane of rabbit cerebral cortex. The current was 20 pA at -60 mV, and the extrapolated zero current potential was -160 mV. Two methods of estimating the effect of pump electrogenicity on resting potential indicated an average contribution of -35 mV. In 20% of the recordings, block of inward rectifier K(+) channels by 10-100 microM Ba(2+) led to a small depolarization, but hyperpolarization was a more common response. Ba(2+) also inhibited depolarization evoked by 20 mM K(+). In arterioles within intact pial membrane, Ba(2+) failed to evoke constriction but inhibited K(+)-induced constriction. The data suggest that cerebral arterioles are vulnerable to the hyperpolarizing effect of the Na(+)/K(+) pump, excessive effects of which are prevented by depolarizing inward rectifier K(+) current

Specialised sympathetic neuroeffector associations in immature rat iris arterioles

Sympathetic nerve-mediated vasoconstriction in iris arterioles of mature rats occurs via the activation of alpha(1B)-adrenoceptors alone, while in immature rat iris arterioles, vasoconstriction occurs via activation of both alpha1- and alpha2-adrenoceptors. In mature rats the vast majority of sympathetic varicosities form close neuroeffector junctions. Serial section electron microscopy of 14 d iris arterioles has been used to determine whether restriction in physiological receptor types with age may result from the establishment of these close neuroeffector junctions. Ninety varicosities which lay within 4 microm of arteriolar smooth muscle were followed for their entire length. Varicosities rarely contained dense cored vesicles even after treatment with 5-hydroxydopamine. 47 % of varicosities formed close associations with muscle cells and 88 % formed close associations with muscle cells or melanocytes. Varicosities in bundles were as likely as single varicosities to form close associations with vascular smooth muscle cells, although the distribution of synaptic vesicles in single varicosities did not show the asymmetric accumulation towards the smooth muscle cells seen in the varicosities in bundles which were frequently clustered together. We conclude that restriction of physiological receptor types during development does not appear to correlate with the establishment of close neuroeffector junctions, although changes in presynaptic structures may contribute to the refinement of postsynaptic responses.