Membrane potential of vascular mono- and multinuclear endothelial cells cultured in vitro

Recent topics on the regulatory mechanism of ecdysteroidogenesis by the prothoracic glands in insects

Molting and metamorphosis are strictly regulated by steroid hormones known as ecdysteroids. It is now widely recognized that ecdysteroid biosynthesis (ecdysteroidogenesis) in the prothoracic gland (PG) is regulated by the tropic factor prothoracicotropic hormone (PTTH). However, the importance of PTTH in the induction of molting and metamorphosis remains unclear, and other mechanisms are thought to be involved in the regulation of ecdysteroidogenesis by the PG. Recently, new regulatory mechanisms, prothoracicostatic factors, and neural regulation have been explored using the silkworm, Bombyx mori, and two circulating prothoracicostatic factors, prothoracicostatic peptide (PTSP) and Bommo-myosuppressin (BMS), have been identified. Whereas PTTH and BMS are secreted from the brain, PTSP is secreted from the peripheral neurosecretory system - the epiproctodeal gland - during the molting stage. The molecular basis of neural regulation of ecdysteroidogenesis has been revealed for the first time in B. mori. The innervating neurons supply both Bommo-FMRF related peptide (BRFa) and orcokinin to maintain low levels of ecdysteroids during the feeding stage. These complex regulatory mechanisms - involving tropic and static factors, peripheral neurosecretory cells as well as the central neuroendocrine system, and neural regulation in addition to circulating factors collaborate to regulate ecdysteroidogenesis. Thus, together they create the finely tuned fluctuations in ecdysteroid titers needed in the hemolymph during insect development.

Histamine-induced inward currents in cultured endothelial cells from human umbilical vein

1. The membrane response to applied histamine of cultured endothelial cells from human umbilical vein was studied by use of whole cell and single channel patch clamp techniques. A value of -27 +/- 1.4 mV was found for the resting potential under whole cell current clamp. No voltage-gated currents were seen at either the macroscopic or single channel levels. 2. At holding potentials of -20 to -40 mV, histamine evoked slow rising, long lasting whole cell inward currents. The inward current was associated with depolarization and decreased input resistance. The calcium ionophore A23187 provoked similar whole cell inward currents. 3. Single channel currents were observed in cell-attached and inside-out patches for both histamine and A23187. The single channel conductance was about 20 pS with a mean open time of 5 ms and a reversal potential of 0 mV in symmetrical potassium solutions. Internal sodium blocked outward going currents. 4. For cell-attached patches, histamine-dependent channel activity required external calcium and was also seen when histamine was present in the bath but not the pipette. Recording from inside-out patches revealed that decreases in 'internal' calcium resulted in the disappearance of channel activity. 5. The histamine-dependent inward current appears to involve calcium-dependent activation of cationic channels.

Voltage-activated potassium, but not calcium currents in cultured bovine aortic endothelial cells

The electrophysiological properties of cultured bovine aortic endothelial cells were characterized using the patch clamp technique. Resting potentials were measured on passing to the whole cell recording configuration and were close to--65 mV in healthy cells. In cell-attached recordings with a high potassium pipette solution, inward single channel currents were observed with zero applied pipette potential. A linear slope conductance of 25 pS was found for a wide range of hyperpolarizing patch potentials and also for depolarizing patch potentials of up to 50-60 mV. A pronounced inward rectification was apparent as no reversal of these currents was seen for larger depolarizations. Whole cell recording in physiological solutions revealed the presence of a hyperpolarization-activated inward current with strong inward rectification and no voltage-dependent ionic current was observed upon depolarization in this subset of cells. Substitution of potassium for external sodium resulted in a shift in the zero current potential consistent with potassium being the main permeant ion. Together with the characteristic voltage-dependent blocking actions of external sodium ions and low concentrations of barium and caesium ions, our results indicate that this current is very similar to the classical inward rectifier as originally described in skeletal muscle and in tunicate eggs. In a second population of cells, a depolarization-activated outward current displaying characteristics of the fast transient A-type potassium current as first reported in molluscan neurones was also observed. No evidence for inward voltage dependent sodium or calcium currents was found.

Calcium entry through receptor-operated channels in bovine pulmonary artery endothelial cells

The activation of endothelial cells by endothelium-dependent vasodilators has been investigated using bioassay, patch clamp and 45Ca flux methods. Cultured pulmonary artery endothelial cells have been demonstrated to release EDRF in response to thrombin, bradykinin, ATP and the calcium ionophore A23187. The resting membrane potential of the endothelial cells was -56 mV and the cells were depolarized by increasing extracellular K+ or by the addition of (0.1-1.0 mM)Ba2+ to the bathing solution. The electrophysiological properties of the cultured endothelial cells suggest that the membrane potential is maintained by an inward rectifying K+ channel with a mean single channel conductance of 35.6 pS. The absence of a depolarization-activated inward current and the reduction of 45Ca influx with high K+ solution suggests that there are no functional voltage-dependent calcium or sodium channels. Thrombin and bradykinin were shown to evoke not only an inward current (carried by Na+ and Ca2+) but also an increase in 45Ca influx suggesting that the increase in intracellular calcium necessary for EDRF release is mediated by an opening of a receptor operated channel. High doses of thrombin and bradykinin induced intracellular calcium release, however, at low doses of thrombin no intracellular calcium release was observed. We propose that the increased cytosolic calcium concentration in endothelial cells induced by endothelium dependent vasodilators is due to the influx of Ca2+ through a receptor operated ion channel and to a lesser degree to intracellular release of calcium from a yet undefined intracellular store.

Lack of selectivity to small ions in paracellular pathways in cerebral and muscle capillaries of the frog

Selectivity to passive permeation of small ions through capillary walls was studied by measurements of diffusion potentials in response to ionic gradients established across capillary walls in frog brain and muscle in superfusion and perfusion experiments. Average dilution potentials in response to 2:1 or 10:1 gradients of NaCl across brain capillaries were 4.2 and 10.2 mV, respectively. The 'diluted' side was negative with respect to the 'undiluted' side, reflecting higher mobility of Cl- than of Na+ ions. Bi-ionic potentials in response to isosmotic KCl:NaCl gradients averaged 5.0 mV in brain capillaries, negative on the KCl side, reflecting higher mobility of K+ than of Na+ ions. The potential variations were symmetrical across the capillary wall. From Planck-Henderson formalism, the relative permeabilities in brain capillaries of Na+, K+ and Cl- were PCl/PNa:1.54 and PK/PNa:1.56, rather close to mobility ratios in free solution. Experiments on muscle capillaries showed similar results to those in brain. Streaming potentials created with excess (200 mosmol) mannitol or sucrose were congruent to 1 mV in brain capillaries and zero in muscle capillaries. It is concluded that the transcapillary permeation pathway in muscle and brain is neutral or weakly charged. The dominant ion permeation is paracellular in all 'continuous' capillaries. The large range of ion permeability of 'continuous' capillaries may be explained by variation in the length of the effective open portion of interendothelial junctions. The very low passive permeability of the blood-brain barrier may be due to an almost closed endothelial junction, leaving only about 0.1% of the length open. The open fraction is functionally similar to that in muscle and mesentery. This interpretation is in accordance with a finite, but very low, permeability to hydrophilic non-electrolytes. Such a brain capillary would still display strong preference for lipid-soluble solutes, but its behaviour cannot be satisfactorily understood from a simple analogy to a cellular plasma membrane.