Radiation effects on biomembranes

Assessing permeability alterations of the blood-bone marrow barrier due to total body irradiation: in vivo quantification with contrast enhanced magnetic resonance imaging

Our aim was to quantify irradiation-induced permeability alterations of the blood-bone marrow barrier (BMB) with dynamic contrast enhanced magnetic resonance imaging (MRI). The standard small molecular contrast agent, gadoterate meglumine, and a new macromolecular contrast agent, carboxymethyldextran-Gd-DOTA (CMD-Gd-DOTA), were compared. Twenty New Zealand white rabbits underwent MRI of the bone marrow before and 1-2 days after total body irradiation (TBI). Dynamic, repetitive T1-weighted MRI was performed before and after injection of either 0.05 mmol/kg BW CMD-Gd-DOTA (n = 10) or 0.5 mmol/kg BW gadoterate (n = 10). Bone marrow contrast enhancement was quantified as delta signal intensity: DeltaSI = |(SIpost - SIpre) / SIpre| * 100%. All MRI data were compared with the histopathologic BMB ultrastructure. Dynamic bone marrow DeltaSI data steadily increased after CMD-Gd-DOTA injection, while blood DeltaSI data slightly decreased. This bone marrow contrast enhancement, indicative of contrast agent extravasation, was significantly higher and prolonged in the irradiated group as compared to non-irradiated controls (P < 0.05) and corresponded to irradiation-induced alterations of the BMB ultrastructure seen on electron microscopy. By contrast, DeltaSI data of non-irradiated and irradiated marrow were not significantly different following gadoterate injection (P > 0.05). We conclude that irradiation-induced alterations in BMB permeability could be reliably assessed with dynamic MRI, using the new macromolecular contrast agent CMD-Gd-DOTA. Bone Marrow Transplantation (2000) 25, 71-78.

Irradiation enhances cellular uptake of carboplatin

PURPOSE

Because radiation is known to damage cellular membranes, the purpose of this study was to determine whether irradiation of cultured cells might modify the cellular uptake of the chemotherapy agent carboplatin.

METHODS AND MATERIALS

Total intracellular platinum was measured using atomic absorption spectrometry in cultured V79 cells and in four Chinese hamster ovary (CHO) cell lines.

RESULTS

Intracellular carboplatin concentrations increased linearly with radiation dose (10-50 Gy) under both hypoxic and oxic irradiation conditions. Similar doses of radiation did not significantly increase the uptake of a nontoxic platinum compound [Pt(NH3)4Cl2.H2O] (p > 0.5). Compared to unirradiated controls, there was no increase in intracellular carboplatin concentrations when carboplatin was irradiated prior to administration to the cell cultures (p > 0.5). Within the 32.5 min or less required to deliver the radiation, a dose of 50 Gy produced approximately a 50% increase in intracellular platinum in V79 cells and approximately an increase of a factor of 1.3-1.6 in the CHO cell lines. Although the increase in drug uptake would be expected to be less than 10% for most cell lines at the doses of radiation used to investigate radiosensitization by carboplatin, this level of increase may play a significant role in the radioenhancement observed in UV41 cells because these excision-repair--deficient cells are much more sensitive to carboplatin as measured by cytotoxicity.

CONCLUSION

These results suggest that some of the enhanced cell killing that results when cells are exposed to carboplatin in combination with radiation may be attributed to an increased cellular uptake. One mechanism of radiopotentiation may be an enhanced chemotoxicity resulting from a radiation-induced increase in carboplatin uptake.

The fusion radiosensitivity of differentiating chick embryo myoblasts in vitro is not determined by the plasma membrane

We have recently demonstrated by dielectric relaxation studies in the radiofrequency range that the sharp drop in the conductivity and permittivity of the membranes of chick embryo myoblasts in vitro, representative of fusion, is either delayed or completely blocked by sublethal doses of ionizing radiation (Santini et al. 1990a). The lowest of the doses investigated (3.25 Gy) caused a 10 h delay in myoblast membrane fusion when the cells were exposed at 24 h of culture, indicating that radiation-induced membrane injury had occurred. The purpose of this study was to determine if the myoblast system under investigation shows the same radiosensitive characteristics if irradiated with 3.25 Gy at various stages of differentiation. Consequently, the myoblasts were exposed to this dose at two different stages of differentiation (12 h or 48 h of culture). We show here that the time at which the myogenic cells are irradiated (state of differentiation) does not seem to affect the magnitude of the fusion delay (which was used as a measure of radiosensitivity of the myoblasts). In fact, the sharp drop in both membrane conductivity and membrane permittivity occurs with the same 10 h delay independent of the time of exposure. The role played by the plasma membrane in determining myoblast response to radiation damage is discussed.

Dose-dependent effects of ionizing radiation on in vitro myoblast fusion

We have recently demonstrated by dielectric relaxation studies in the radio-frequency range that there is a sharp decrease in the conductivity and permittivity of the membranes of chick embryo myoblasts in vitro at the time of fusion (60 h) (Bonincontro et al. 1987). This sharp fall in membrane electrical parameters was subsequently shown to be due to changes in ionic flux, particularly of the Na+/K+ equilibrium (Santini et al. 1988). Ionizing radiation induces a wide variety of effects on biological membranes, including variations in membrane ionic transport. We wished to investigate if sublethal doses of gamma-irradiation could affect membrane electrical parameters and thus myoblast membrane fusion. Consequently, chick embryo myoblast aggregate cultures were irradiated with 3.25, 5.15 or 6.35 Gy at 24 h of culture. We found that the lower dose delays membrane fusion by about 10 h while the two higher doses block fusion up to 120 h of culture. Aggregates showed a very high cell viability. The possible mechanisms by which ionizing radiation causes these variations in myoblast membrane electrical properties and fusion are discussed.

Effects of ionizing radiation on artificial (planar) lipid membranes. I. Radiation inactivation of the ion channel gramicidin A

The ion channel formed by the pentadecapeptide gramicidin A in planar lipid membranes is extremely sensitive to ionizing radiation. The membrane conductance may drop by several orders of magnitude under appropriate experimental conditions (low pH and presence of oxygen). The radiation sensitivity is strongly reduced for gramicidin M-. This analogue has the four tryptophan residues replaced by phenylalanines. Experiments performed in the presence of various radical scavengers suggest that the inactivation of the channel is due to a combined action of OH and of HO2 radicals at the tryptophan residues. The shape of the inactivation curves following continuous radiolysis or pulse radiolysis were found to be in fair agreement with a simple model which assumes that the damage of a single tryptophan residue is sufficient for channel inactivation. The conductance of inactivated channels could not be resolved within the experimental accuracy. This is contrary to photolysis of gramicidin channels found by Busath and Waldbilling (1983), where a broad distribution of low conductance states was observed. The inactivation by radiolysis seems to represent an 'all-or-none-process' of the channel conductance.

Dose-response studies of single dose ionizing radiation on the ciliated epithelium of the trachea of the rabbit. A physiologic and ultrastructural investigation

The effects of ionizing radiation on the ciliated epithelium of the rabbit's trachea were studied by ultrastructural and physiologic examinations during 10 days after single doses of 2 to 30 Gy. Low doses (2 and 2.5 Gy) caused during the first days after irradiation increased ciliary activity which during the following days returned to normal. Ultrastructurally blebs were observed on the cilia. Five and 10 Gy caused slight increment of the ciliary activity, also more marked structural effects with swollen ciliary tips, bent tips and some cilia broken and adhering to each other. Larger single doses (15-30 Gy) caused reduced and irregular ciliary activity; ultrastructurally, most cilia were broken and adhered to each other, but some cilia seemed to be unaffected. The goblet cells increased in number already after 2 and 2.5 Gy. Mucinous granula could be observed on the ciliary surface and after 30 Gy there was an explosive emptying of the goblet cells.

Response of ciliated cells during irradiation at different dose rates

Early radiation effects on the physiologic activity of ciliated cells during exposure have been investigated at 0.34Gy/s and 0.05 Gy/s. The results obtained indicate a more intense early biologic effect at a higher dose rate. The mechanisms of radiation injury are discussed.