Effects of Metabolic Inhibition and Acidosis on the Contractility of the Pulmonay and Ear Artery

Hypoxia causes contraction in pulmonary artery, whereas it causes relaxation in systemic artery. The purpose of this study is to test whether pulmonary artery would respond to metabolic inhibition and acidosis differently from ear artery. Rabbit pulmonary artery and ear artery were precontracted with phenylephrine or KCI, and then exposed to metabolic blockers (dlnttrophenof Hrlvl'), Na-cyanide (NaCN)) and acidosis. Contractile forces of ear artery induced by 30mM KCI and 1O-6M phenylephrine were 2-3times(n =7) and 5-9 times (n = 7) larger than that of the pulmonary artery, respectively, DNP and NaCN produced a dose-dependent relaxation in the pulmonary and ear artery, and the relaxation was more profound in the ear artery than in pulmonary artery. This effect was independent of the presence of the endothelium. Extracellular acidosis reduced the tone of the KCI-induced contraction, more in the ear artery than in pulmonary artery. These results indicate that pulmonary artery is more resistant to both of the inhibition of metabolism and acidosis than ear artery

Ionic mechanisms and Ca2+ dynamics underlying the glucose response of pancreatic β cells: a simulation study

To clarify the mechanisms underlying the pancreatic β-cell response to varying glucose concentrations ([G]), electrophysiological findings were integrated into a mathematical cell model. The Ca2+ dynamics of the endoplasmic reticulum (ER) were also improved. The model was validated by demonstrating quiescent potential, burst–interburst electrical events accompanied by Ca2+ transients, and continuous firing of action potentials over [G] ranges of 0–6, 7–18, and >19 mM, respectively. These responses to glucose were completely reversible. The action potential, input impedance, and Ca2+ transients were in good agreement with experimental measurements. The ionic mechanisms underlying the burst–interburst rhythm were investigated by lead potential analysis, which quantified the contributions of individual current components. This analysis demonstrated that slow potential changes during the interburst period were attributable to modifications of ion channels or transporters by intracellular ions and/or metabolites to different degrees depending on [G]. The predominant role of adenosine triphosphate–sensitive K+ current in switching on and off the repetitive firing of action potentials at 8 mM [G] was taken over at a higher [G] by Ca2+- or Na+-dependent currents, which were generated by the plasma membrane Ca2+ pump, Na+/K+ pump, Na+/Ca2+ exchanger, and TRPM channel. Accumulation and release of Ca2+ by the ER also had a strong influence on the slow electrical rhythm. We conclude that the present mathematical model is useful for quantifying the role of individual functional components in the whole cell responses based on experimental findings

Effects of Calcium Channel Blockers on Sodium-Free Contracture in Atrial Muscle of the Rabbit

Effects of organic and inorganic Ca-channel blockers on Na-Ca exchange system were investigated in the rabbit atrial muscle. Atrial muscle strips were perfused with Kr-free Tyrode solution in order to depress the sodium pump activity. Removal of external sodium(sodium being replaced by Tris) induced a contracture which reached a maximum after 1 min and effects of Ca-channel blockers on the magnitude of contracture were analysed. The results obtained were as follows: 1. In the concentrations of 30 pM, 100 pM and 300 ,uM Mn2 + increased the magnitude of Na-removal contracture, but decreased it in the concentration above 2 ,uM. Verapamil(IO-6 M) pretreatment did not alter the effect of Mn2+ on sodium-removal contracture. 2. La3+, as Mn2+, increased the magnitude of contracture in the concentrations of 30-300 pM, and decreased the contracture in higher concentrations(> 1 mM) more prominently than Mn2+ did. 3. 0-600 also increased the contracture in the concentrations of 5 x 10-8 M, 10-7 M and 5 x 10-7 M but had no effect in higher concentratlonsfl Orf - 10-5 M), On the other hand diltiazem had no dffect on the contracture in a wide range of concentrations (up to 10-4 M). From the above results, it is concluded that Mn2+, La3+ and 0600 in lower concentrations stimulate the Na-Ca exchange system, whereas, Mn2+, La3+ and 0-600 in higher concentrations depress the exchange system and that Na-Ca exchange might be regulated by Ca-channel blockers and this regulation is sensitive to the concentration of Ca-channel blockers

The Effect of Stimulation Frequency on the Ionic Currents in Single Atrial Cells of the Rabbit

In single atrial cells isolated from rabbit hearts the calcium current and [Caj-dependent transient outward current were recorded using the whole-cell clamp technique and the effect of stimulation frequency on these currents was investigated. Voltage dependent transient outward current, which contributes the initial, rapid repolarization phase of the action potential and is frequency-dependent, was also investigated. Increasing the stimulation frequency from O. 025 Hz to 1 Hz had no effect on the calcium current and [Caj-dependent transient outward current and greatly inhibited voltage-dependent transient outward current. The amplitude of voltage dependent transient outward current increased as the membrane potential became depolarized, its steady-state inactivation spans the voltage range -70 mV to -10 mVand steady-state activation curve -30 mV to 30 mV. Within the range of the resting membrane potential (at -70 mV), the voltage-dependent recovery time constant was 1. 3 s. The reversal potential was about -50 mV. Voltage-dependent transient outward current was inhibited by K-channel blockers and not inhibited by modulation of [Cali. From the above findings, it is concluded that due to the amplitude and voltage-dependent recovery time constant which were the basic mechanisms for frequency-dependency, the voltage- dependent transient outward current contributes the initial, rapid repolarization phase and changed the action potential configuration according to stimulation frequency in the rabbit atrium

Mechanosensitive activation of K+ channel via phospholipase C-induced depletion of phosphatidylinositol 4,5-bisphosphate in B lymphocytes

In various types of cells mechanical stimulation of the plasma membrane activates phospholipase C (PLC). However, the regulation of ion channels via mechanosensitive degradation of phosphatidylinositol 4,5-bisphosphate (PIP(2)) is not known yet. The mouse B cells express large conductance background K(+) channels (LK(bg)) that are inhibited by PIP(2). In inside-out patch clamp studies, the application of MgATP (1 mm) also inhibited LK(bg) due to the generation of PIP(2) by phosphoinositide (PI)-kinases. In the presence of MgATP, membrane stretch induced by negative pipette pressure activated LK(bg), which was antagonized by PIP(2) (> 1 microm) or higher concentration of MgATP (5 mm). The inhibition by PIP(2) was partially reversible. However, the application of methyl-beta-cyclodextrin, a cholesterol scavenger disrupting lipid rafts, induced the full recovery of LK(bg) activity and facilitated the activation by stretch. In cell-attached patches, LK(bg) were activated by hypotonic swelling of B cells as well as by negative pressure. The mechano-activation of LK(bg) was blocked by U73122, a PLC inhibitor. Neither actin depolymerization nor the inhibition of lipid phosphatase blocked the mechanical effects. Direct stimulation of PLC by m-3M3FBS or by cross-linking IgM-type B cell receptors activated LK(bg). Western blot analysis and confocal microscopy showed that the hypotonic swelling of WEHI-231 induces tyrosine phosphorylation of PLCgamma2 and PIP(2) hydrolysis of plasma membrane. The time dependence of PIP(2) hydrolysis and LK(bg) activation were similar. The presence of LK(bg) and their stretch sensitivity were also proven in fresh isolated mice splenic B cells. From the above results, we propose a novel mechanism of stretch-dependent ion channel activation, namely, that the degradation of PIP(2) caused by stretch-activated PLC releases LK(bg) from the tonic inhibition by PIP(2)

Localization and function of the renal calcium-sensing receptor

The ability to monitor changes in the ionic composition of the extracellular environment is a crucial feature that has evolved in all living organisms. The cloning and characterization of the extracellular calcium-sensing receptor (CaSR) from the mammalian parathyroid gland in the early 1990s provided the first description of a cellular, ion-sensing mechanism. This finding demonstrated how cells can detect small, physiological variations in free ionized calcium (Ca 2+) in the extracellular fluid and subsequently evoke an appropriate biological response by altering the secretion of parathyroid hormone (PTH) that acts on PTH receptors expressed in target tissues, including the kidney, intestine, and bone. Aberrant Ca 2+ sensing by the parathyroid glands, as a result of altered CaSR expression or function, is associated with impaired divalent cation homeostasis. CaSR activators that mimic the effects of Ca 2+ (calcimimetics) have been designed to treat hyperparathyroidism, and CaSR antagonists (calcilytics) are in development for the treatment of hypercalciuric disorders. The kidney expresses a CaSR that might directly contribute to the regulation of many aspects of renal function in a PTH-independent manner. This Review discusses the roles of the renal CaSR and the potential impact of pharmacological modulation of the CaSR on renal function