Biomedical subjects

R J Cork

Publications and source records attributed to R J Cork.

24 records · Page 2Linked to original sources

PubMed1990

Computer-aided analysis of polarized neurite growth. Effects of applied electrical fields on neuronal development.

A computer-based system to record and analyze the polarized growth of neurons growing in culture has been developed. The first application of this system was to study the effects of electrical fields on the growth of isolated Xenopus neurons. Simple coordinate data are collected from photographic negatives of isolated neurons, using a digitizing tablet. Morphometric parameters for individual cells and whole neuronal populations are calculated. A variety of indicators of polarized growth are defined and we compare our methods with those used in previous studies, and the most appropriate methods for describing polarized growth are discussed. Although these methods have been designed specifically to assess the effects of electrical fields on neurite growth, they could be applied to other experiments where biased growth of neurites is expected. They could be used in studies of the tropistic effects of growth factor gradients, or variations in the substrate. Alternatively, they could be useful for the analysis of the many other types of polarized growth found in a wide range of cell types.

Animals↗

PubMed1990

H-ras(val12) induces cytoplasmic but not nuclear events of the cell cycle in small Xenopus oocytes.

Microinjection of H-ras(val12) protein into fully grown Xenopus oocytes has been shown to induce meiotic maturation. In the present study, mRNA encoding the mutant ras protein was injected into both fully grown (stage 6) and growing (stage 4) oocytes. The mRNA induced nuclear breakdown in stage 6 oocytes, as expected. However, the mRNA induced neither nuclear breakdown nor maturation promoting factor when injected into stage 4 oocytes. Instead, the response in stage 4 oocytes included an activation pulse of calcium, cortical granule breakdown, elevation of the vitelline envelope, and abortive cleavage furrows, all of which are characteristics of the activation response in mature eggs. In addition, the injected mRNA led to increased rates of endogenous protein synthesis and the migration of subcortical organelles into the oocyte interior. These observations are discussed relative to the suggestion that oncogenic ras protein leads to an increase in both diacylglycerol and inositol trisphosphate, which then regulate the various cytoplasmic events described.

Animals↗

PubMed1990

The distribution of free calcium in transected spinal axons and its modulation by applied electrical fields.

Intracellular free-calcium concentration ([Ca2+]i) was measured in lamprey spinal axons using the fluorescent calcium indicator fura 2. We used both a photomultiplier tube and a video-image processing system to measure the temporal and spatial distributions of [Ca2+]i in the proximal segments of transected axons. Within 3 min following transection, a spatially graded increase in the [Ca2+]i was apparent in the last few millimeters of the axons. Superimposed on the initial gradient was a moving front of calcium that progressed up the axon, reaching 1.6 mm from the cut end in 3 hr. The [Ca2+]i behind the moving front exceeded 10 microM. This movement of Ca2+ was greatly reduced by an externally applied electrical field with the cathode distal to the lesion and was increased by an applied field of the opposite polarity. When axons were transected in Ca2(+)-free medium, no increases in [Ca2+]i occurred. One d after transection, [Ca2+]i was at or below the precut levels, except in the distal 250 microns, where it remained slightly elevated. Therefore, axons can survive the high levels of [Ca2+]i that occur after transection and can reestablish normal [Ca2+]i levels within 24 hr. Measurements of both the diffusion coefficient and the fluorescence polarization of fura 2 indicate that the dye is not significantly bound to axoplasmic components.

Animals↗

PubMed1987

A rise in cytosolic calcium is not necessary for maturation of Xenopus laevis oocytes.

Cytoplasmic free calcium levels during progesterone-induced meiotic maturation in Xenopus laevis oocytes were measured using the photoprotein aequorin. The resting level of [Ca2+]i was 92.6 +/- 30 nM. No significant changes were observed after progesterone addition, although a large pulse of [Ca2+]i was observed upon activation of matured oocytes. These findings are discussed in terms of the role of calcium in maturation and it is concluded that calcium is not the second messenger for progesterone. This conclusion is further supported by the finding that 100 microM TMB-8, a blocker of intracellular calcium release, had no effect on progesterone-induced maturation.

Aequorin↗

PubMed1987

Calcium does not act as a second messenger for adrenergic and cholinergic agonists in corneal epithelial cells.

The role of changes in intracellular [Ca2+]i as a second messenger in response to either adrenergic or cholinergic agonists was determined in isolated bovine corneal epithelial cells. [Ca2+]i was measured in suspensions of cells loaded with either of the fluorescent indicators quin2 or indo-1, as well as in single cells loaded with fura-2. Fluorescence from the cell suspensions was measured in a spectrofluorometer while single cell fluorescence was measured using a modified fluorescence microscope with a photon counting photometer. Cells were loaded with these dyes by incubation in Ringer's (pH 8.1) containing 2-50 microM of the acetoxymethyl ester of the indicator. Fluorescence was measured before and after exposure to either, one of the adrenergic agonists isoproterenol, phenylephrine or epinephrine, or the cholinergic agonist carbachol. The resting [Ca2+]i level from the quin2 experiments was 115 nM +/- 41 nM (SEM) (n = 23) whereas with fura-2 it was 71 +/- 10 nM (n = 30). In no case did we see any change in [Ca2+]i within 15 min after addition of any agonist but we were able to observe increased calcium when 0.5 microM ionomycin was added to either the same or untreated cells. The disparity in the resting levels determined by the two methods may result from various calibration problems. Our results indicate that changes in [Ca2+]i have no second messenger role in response to these agonists.

Aminoquinolines↗

PubMed1979

Vesicles in guard-cell walls and their possible roles in the stomatal mechanism.

A variety of vesicular inclusions have been observed in guard-cell walls. They have been seen in a number of species and are mostly membrane-bound. They appear confined to the upper and ventral walls of the guard cell. A number of possible origins and functions are discussed. They may be involved in wall deposition or cuticle formation possibly in a role similar to ectodesmata. They also may serve to increase the wall-plasmalemma interface to enhance the movement of ions during the stomatal mechanism.

Cell Wall↗