Simultaneous oscillations in the membrane potential of pig coronary artery endothelial and smooth muscle cells

Determination of gap junctional intercellular communication by capacitance measurements

Electrical coupling between cells is usually measured using the double-patch-clamp technique with cell pairs. Here, a single patch-clamp technique that is not limited to cell pairs is described to determine electrical coupling between cells. Capacitance measurements in clusters of normal rat kidney (NRK) fibroblasts were used to study intercellular communication. In the whole-cell patch-clamp configuration capacitive transients were evoked by applying small voltage pulses. Total membrane capacitance was calculated from these capacitive transients after determination of access resistance, membrane conductance, and the decay constant of the transients, or alternatively by integrating the current transient. We found that in clusters of one to ten cells, membrane capacitance increased linearly with cell number, showing that the cells are electrically coupled. Membrane conductance of the cluster of cells also increased, as expected for cells that are well coupled. In subconfluent and confluent cultures, high membrane conductances together with large capacitive transients were observed, indicative of electrical coupling. Capacitance could only be determined qualitatively under these conditions, due to space clamp problems. In the presence of the gap junctional inhibitors halothane, heptanol or octanol, capacitance of all clusters of cells fell to single-cell levels, showing a complete uncoupling of the cells. The tumour promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) also uncoupled the cells completely, with 10 min. We conclude that capacitance measurements can provide a useful tool to study changes in intercellular communication in clusters of cells.

Role of endothelium in oscillatory contractile responses to various receptor agonists in isolated small mesenteric and epicardial coronary arteries

Employing the isometric tension recording method, we studied the role of endothelium or endothelium-derived relaxing factor (EDRF) in the generation of rhythmic oscillations observed in contractile responses to various receptor agonists in isolated rabbit small mesenteric and epicardial coronary arteries. Norepinephrine (NE, 0.1-10 microM) generated oscillatory contraction in endothelium-intact strips from the mesenteric arteries. Similarly, acetylcholine (ACh, 10 microM), histamine (10 microM) and serotonin (10 microM) generated oscillatory contraction in endothelium-intact strips from the coronary arteries. These agonist-induced oscillations in both arteries were consistently eliminated by either endothelial denudation or EDRF pathway inhibitors including NG-nitro L-arginine (30 and 100 microM), oxyhemoglobin (3 and 10 microM) and methylene blue (3 and 10 microM). In contrast, EDRF releasers such as ACh or A23187 augmented the oscillations in the endothelium-intact strips. SNP (0.03-30 microM) failed to generate oscillations in NE (10 microM)-preconstricted endothelium-denuded strips from the mesenteric arteries. In conclusion, these agonist-induced oscillations are probably mediated through EDRF. The inability of SNP to generate oscillations suggests the obligatory role of the endothelium in generation of the oscillations. The oscillatory release of EDRF by endothelial cells may be responsible for generation of the oscillations.

Effect of 5-hydroxytryptamine on the membrane potential of endothelial and smooth muscle cells in the pig coronary artery

1. Many endothelium-dependent vasodilators hyperpolarize the endothelial cells in blood vessels. It is not known whether these hyperpolarizations are linked to nitric oxide synthesis or to an endothelium-derived hyperpolarizing phenomenon, since most of the vasodilators release both factors. In this context, we first verified that the endothelium-dependent relaxations induced by 5-hydroxytryptamine (5-HT) on pig coronary arteries are due only to the activation of the nitric oxide pathway. Then we studied the effects of 5-HT on membrane potential of endothelial and smooth muscle cells. 2. In the absence of endothelium, 5-HT caused a concentration-dependent contraction of coronary artery strips. No change of the smooth muscle cell membrane potential was observed during contraction to 1 microM 5-HT. 3. In the presence of 1 microM ketanserin to suppress the contractile effect of 5-HT, 5-HT induced concentration-dependent relaxation of endothelium-intact strips precontracted by 10 microM prostaglandin F2 alpha (PGF2 alpha). These relaxations were suppressed by 1 microM NG-nitro-L-arginine, an inhibitor of nitric oxide synthesis, showing that they were produced predominantly by nitric oxide. 4. In the presence of 1 microM ketanserin, 1 microM 5-HT did not change the smooth muscle cell membrane potential of strips precontracted by either 10 microM PGF2 alpha or by 10 microM acetylcholine (ACh). In the same conditions, 1 microM 5-HT caused a weak 2.6 +/- 0.4 mV hyperpolarization, of the endothelial cells. 5. In conclusion, the fact that 5-HT did not change the membrane potential of smooth muscle cells and only weakly hyperpolarized the endothelial cells during relaxations, suggests that in both cell types no electrical events accompany activation of the nitric oxide pathway. This is in contrast to the hyperpolarizations observed in endothelial and smooth muscle cells when the endothelium-derived hyperpolarization factor (EDHF) pathway is activated.

Spontaneous and 5HT-induced cyclic contractions in superficial temporal arteries from chronic and episodic cluster headache patients

Superficial temporal arteries (STAs) are abnormally dilated in the painful side during cluster headache (CH) attacks. We have assessed the possible dysfunction of these arteries by comparing in vitro the reactivity of STAs removed from the painful side of CH patients during a cluster of attacks with that of STAs from patients free of CH. The responses to KCl and norepinephrine (NE) of both types of arteries were similar. Serotonin (5HT) induced a classical dose-dependent constriction in arteries from non-CH patients, but systematically triggered rhythmic contractions in arteries from episodic CH patients. Arteries from chronic CH patients also showed spontaneous rhythmic contractions. In both cases, this activity was stopped by calcitonin gene-related peptide (CGRP) but, even in the presence of CGRP, it could be restored by low concentrations of 5HT. Thus, 5HT, unlike NA, can trigger rythmic activity in STAs of CH patients and may play a major role in CH through abnormal smooth muscle cell reactivity.

Acetylcholine-stimulated changes of membrane potential and intracellular Ca2+ concentration recorded in endothelial cells in situ in the isolated rat aorta

The intracellular free Ca2+ concentration and membrane potential changes evoked by acetylcholine were recorded from whole-cell patch-clamped endothelial cells in situ in the isolated rat aorta. The endothelium had a resting membrane potential of -52 +/- 3 mV (SEM, range -35 mV to -76 mV n = 34) and a low input resistance (32 - 54 M omega). The membrane potential hyperpolarised by 3-30 mV on continuous application of acetylcholine at concentrations that produced endothelium-dependent relaxations in isolated rat aortic rings (range 1-500 nM). The response often comprised complex fluctuations of hyperpolarised membrane potential. Calcium concentration was measured with the fluorescent indicator furaptra, which has a wide range and minimises Ca2+ buffering. Acetylcholine evoked an initial rapid elevation of intracellular Ca2+ concentration, peaking in the range 6-35 microM, which declined with a half time of approximately 6 s, followed by repetitive [Ca2+] spikes of amplitude 2-18 microM in 23 of 34 cells. The initial [Ca2+] transient and hyperpolarisation were unaffected by removal of external Ca2+, whilst subsequent [Ca2+] spikes and maintained hyperpolarisations required the presence of external Ca2+. Both the hyperpolarisation and Ca2+ responses elicited by acetylcholine were abolished by atropine (1 microM). These results show that endothelial cells in situ exhibit large, fast repetitive [Ca2+] spikes in response to extracellular acetylcholine.

Regulation of blood-brain barrier endothelial cells by nitric oxide

Nitric oxide (NO) synthesized by vascular endothelial cells is a potent vasodilator substance. The actions of NO extend well beyond its vasodilatory properties, and increasingly, NO has been recognized as an important signal for intercellular and intracellular communication. Recently, NO has been implicated in the regulation of vascular and blood-brain barrier permeability. NO has also been shown to modulate ion channels in excitable cells, thus affecting neuronal firing. We report the results of patch-clamp experiments that show a modulatory action of NO as well as cGMP and cAMP on a hyperpolarization-activated current (Iha) carried by both Na+ and K+ ions in blood-brain barrier endothelial cells. Iha was recorded in cells dialyzed with 0.2 mmol/L GTP-gamma-S to inhibit a large inwardly rectifying potassium current. This ionic current and its modulation by NO may play a role in the regulation of the transport of ions, nutrients, and other molecules to the brain and serve as an integral part of the blood-brain barrier. The modulation of Iha by a cyclic guanosine nucleotide may also explain previous reports suggesting a role for NO in the regulation of blood-brain barrier function.

Cell-to-cell communication coordinates blood flow control

The control of tissue blood flow is a dynamic process exemplified by the interaction among physical, chemical, and electrical events occurring within the vessel wall and between the vasculature and tissue parenchyma. The range of blood flow control achieved in vivo is illustrated by functional hyperemia in exercising skeletal muscle: maximal flow can exceed resting values by more than 50-fold. Blood flow control is integrated among many vessel segments, beginning with resistance arteries external to the muscle and encompassing the arteriolar network within the muscle. As metabolic demand increases, the locus of blood flow control shifts from distal arterioles, which control capillary perfusion and blood flow distribution within the tissue, to the proximal arterioles and resistance arteries, which control the total volume of flow into the muscle. A fundamental question centers on how this vasomotor activity is actually coordinated throughout the resistance network. The interaction within and among vascular segments can be explained by chemical and electrical signals to smooth muscle cells (SMCs) and endothelial cells (ECs) in response to changes in transmural pressure as well as luminal shear stress. Increasing pressure results in SMC contraction via the myogenic response. Increasing flow stimulates ECs to release autacoids (eg, nitric oxide), which relax SMCs. Pressure and flow thereby provide opposing mechanical stimuli that interact in the maintenance of vasomotor tone throughout the resistance network. Vasomotor signals are also conducted along arterioles through cell-to-cell coupling between ECs and SMCs, thereby coordinating vasomotor activity of cells within a branch and among branches.(ABSTRACT TRUNCATED AT 250 WORDS)