Ionophoretic Activity Research Articles

Insulinotropic effects of hypoglycaemic and hyperglycaemic sulphonamides: the ionophoretic hypothesis.

Hypoglycaemic sulphonamides stimulate net uptake of 45Ca++ and insulin release in isolated pancreatic islets. These effects are antagonized by organic calcium-antagonists (e.g. suloctidil). In an artificial system, hypoglycaemic sulphonamides, such as gliclazide, stimulate the translocation of calcium into or across a hydrophobic immiscible domain, a process enhanced by the antibiotic ionophore A 23187 and antagonized by suloctidil. In this artificial system, the A 23187-mediated process of calcium countertransport is stimulated by gliclazide and inhibited by diazoxide. It is postulated that the insulinotropic action of hypoglycaemic and hyperglycaemic sulphonamides is primarily attributable to the ionophoretic action of these drugs.

Ionophoretic activity in pancreatic islets

Extracts of pancreatic islets stimulate the translocation of calcium from an aqueous into an organic immiscible phase. This ionophoretic activity, which is derived mainly from membrane-rich subcellular fractions, displays several features in common with that of A23187 in the same model. The phenomenon of calcium translocation caused by either the islet extract or the antibiotic ionophore represents a power function of the concentration of ionophoretic material; it is saturable at high calcium concentrations, affected by the concentration of Na + and pH of the aqueous phase, increased at low temperature, and inhibited by suloctidil, the latter inhibitory effect being antagonized by calcium itself. These findings underline the potential significance of native ionophores in the regulation of calcium movements across membrane systems in the islet cells.

Calcium ionophore activity of a prostaglandin B 1 derivative (PGB X)

A water soluble derivative of PGB 1, designated PGB X, has been found to stimulate the release of Ca 2+ from fragmented sarcoplasmic reticulum and heart mitochondria; its activity is almost two orders of magnitude greater than other prostaglandins. PGB X demonstrates ionophoretic activity in its ability to transfer Ca 2+ from an aqueous to an organic phase.

Biological function of gramicidin: selective inhibition of RNA polymerase.

This paper describes a novel biochemical effect of gramicidin, a class of peptide antibiotics produced by Bacillus brevis during the transition from vegetative growth to sporulation. Gramicidin inhibits RNA synthesis by purified RNA polymerase (nucleosidetriphosphate:RNA nucleotidyl-transferase, EC 2.7.7.6) by interfering with the binding of RNA polymerase to DNA. This effect seems to involve the destabilization of the "open" RNA polymerase-DNA complex, a mode of action consistent with the control of promoter selection. Selectivity in the inhibition of RNA synthesis by gramicidin is observed when transcription is partially blocked by low levels of actinomycin D. Since the inhibition of RNA synthesis by gramicidin is obtained in a highly purified system devoid of membranes, it must be distinct from the ionophoretic activity of the antibiotic. It is possible that this new mode of action reflects the function of gramicidin during bacterial sporulation.

Effects of prostaglandins and oxytocin on calcium release from a uterine microsomal fraction.

A microsomal fraction resembling striated muscle sarcoplasmic reticulum was isolated from uterine smooth muscle. ATP induces calcium accumulation in this fraction. Increased temperature enhances calcium accumulation and calcium-activated ATPase. In the absence of ATP, approximately 35% of the intrinsic calcium exchanges with the 45Ca in the incubation medium. In the presence of ATP, exchange of intrinsic calcium with 45Ca increases by an amount which equals the ATP-dependent calcium binding. In preparations partially preloaded with calcium, a steady state of bound calcium is reached when the ATP is exhausted. Calcium is released under these conditions by prostaglandins E2 and F2alpha, but not by PGF1beta. The antibiotic ionophores X537A and A23187, as well as oxytocin, also release calcium previously accumulated under ATP stimulation. None of these agents, with the exception of oxytocin, release intrinsic calcium. Thus, the effect of prostaglandins resembles that of the ionophores, suggesting an ionophoretic action of these prostaglandins. The release of calcium conforms with the in vivo smooth muscle contracting action of these agents.

The effect of A23187 ionophore on calcium movements and contraction processes in single barnacle muscle fibres.

1. Ca movements were studied in single giant muscle fibres of the barnacle before and after exposure to the Ca ionophore A23187. 2. In fibres micro-injected with the photoprotein aequorin, the resting rate of light emission (resting glow) reversibly increased when the external Ca was augmented in the presence of A23187. This resulted from an increased Ca influx through the outer membrane. In zero external Ca, A23187 induced a delayed increase of the resting glow which was related to Ca leakage from intracellular storage sites. 3. When the experiment was carried out at 5 degrees C instead of 22 degrees C, the resting glow about doubled (temperature-sensitive mechanisms for maintaining a low myoplasmic Ca2+ were thus depressed) but A23187 was much less effective in increasing Ca influx in high external Ca. 4. In 45Ca efflux experiments, A23187 increased both the Ca-Ca exchange diffusion and the net Ca outflux at the outer membrane of the fibre. The Na-Ca exchange process was not affected to a significant extent. 5. The Ca2+ transient and the force of the contraction elicited by imposed membrane depolarizations (below the threshold of the propagated action potential) were markedly increased by A23187, but the electromechanical threshold was not significantly modified. The linear relation between membrane depolarizations and the area of the Ca transient became much steeper in the presence of A23187. However the relation between the area of the Ca transient and the force output was not affected which suggests that A23187 specifically involved the Ca2+ intracellular release, but not the subsequent Ca-troponin reaction. 6. A23187 potentiates by ionophoretic action the Ca movements at the outer membrane, and it also acts inside the intact muscle fibre to intensify the intracellular release of Ca.

Phospholipids as ionophores.

The ionophoretic capabilities of phospholipids have been examined by direct measurement in a Pressman cell of the phospholipid-mediated translocation of cations across an organic phase separating two aqueous phases. Cardiolipin and phosphatidic acid were the most active inonophores among the phospholipids tested, with activities comparable to that of X537A in respect to the translocation of divalent cations. Cardiolipin translocates both divalent and monovalent cations at approximately equal rates. The ionophoretic activity of cardiolipin could be modulated by other phospholipids (inhibition), by butacaine (stimulation), by complexation with cytochrome c (inhibition), and by ruthenium red and lanthanum (inhibition). The rate of translocation of cations mediated by cardiolipin was independent of pH over a wide pH range (5.4 to 8.3). The same general pattern of properties observed for cardiolipin applied to phosphatidic acid except for stimulation by butacaine. Complexation of phospholipid mixtures, such as asolectin or mitochondrial lipid, with reduced cytochrome c, enhanced the ionophoretic capability of these phospholipids by 1 order of magnitude. The complex thus formed has the properties of a polyionophore. The possible physiological significance of this enormous ionophoretic potential of phospholipids is examined.