Membrane protein diffusion sets the speed of rod phototransduction

Retinal rods signal the activation of a single receptor molecule by a photon. To ensure efficient photon capture, rods maintain about 109 copies of rhodopsin densely packed into membranous disks. But a high packing density of rhodopsin may impede other steps in phototransduction that take place on the disk membrane, by restricting the lateral movement of, and hence the rate of encounters between, the molecules involved. Although it has been suggested that lateral diffusion of proteins on the membrane sets the rate of onset of the photoresponse, it was later argued that the subsequent processing of the complexes was the main determinant of this rate. The effects of protein density on response shut-off have not been reported. Here we show that a roughly 50% reduction in protein crowding achieved by the hemizygous knockout of rhodopsin in transgenic mice accelerates the rising phases and recoveries of flash responses by about 1.7-fold in vivo. Thus, in rods the rates of both response onset and recovery are set by the diffusional encounter frequency between proteins on the disk membrane.

Systemic administration of phenyl-N-tert-butylnitrone protects the retina from light damage

PURPOSE

This study was conducted to test the hypothesis that phenyl-N-tert-butylnitrone (PBN), a spin-trapping agent known to cross the blood-brain barrier and protect the brain from ischemia-reperfusion injury, is incorporated into the retina after intraperitoneal injection and protects photoreceptor cells from the damaging effects of constant visible light.

METHODS

Albino rats were injected intraperitoneally with PBN (aqueous solution) or water, or were not injected, and then were placed in constant light (2700 lux) for 24 hours. The incorporation of PBN into the retina was determined by high-performance liquid chromatography. Electroretinograms (ERGs) were recorded before light treatment and 1 and 15 days after the cessation of exposure to constant light. Eyes were taken for histology at each time point and outer nuclear layer (ONL) thickness determined.

RESULTS

PBN was incorporated into the retina after intraperitoneal injection. Both control (water-injected and uninjected) groups exposed to constant light maintained only 28% of ONL thickness and 20% of retinal function, compared with the unexposed control group. In contrast, the PBN-treated animals maintained 80% of ONL thickness and exhibited 87% of retinal function.

CONCLUSIONS

PBN protects the albino rat retina from the damaging effects of constant light stress. That light-induced and hereditary retinal degenerations share certain morphologic hallmarks and follow a similar apoptotic mechanism of degeneration raises the possibility of pharmacologic therapy for hereditary and environmentally induced neurodegenerative disorders.

Increase in serum S100A1-B and S100BB during cardiac surgery arises from extracerebral sources

BACKGROUND

Elevated levels of serum S100B after coronary artery bypass grafting may arise from extracerebral contamination. Serum S100B content was analyzed in several tissues, and the two dimers S100A1-B and S100BB were analyzed separately in blood.

METHODS

Serum, shed blood, marrow, fat, and muscle were studied in patients undergoing coronary artery bypass grafting with cardiopulmonary bypass using suction either to the cardiotomy reservoir (group 1, n = 10) or to a cell-saving device (group 2, n = 10), or operated on off-pump (group 3, n = 10).

RESULTS

Serum S100B was sixfold higher in group 1 than in groups 2 and 3, which were identical. The same ratio between S100A1-B and S100BB was found in all groups. When compared with serum, S100B was 10(2) to 10(4) times higher in marrow, fat, muscle tissue, and shed blood.

CONCLUSIONS

Separate analysis of S100A1-B and S100BB did not distinguish between S100B of cerebral and extracerebral origin. The concept that S100B only originates in astroglial and Schwann cells is wrong. Fat, muscle, and marrow in mediastinal blood contain high levels of S100B. Cardiopulmonary bypass caused no increase in S100B.

Regulation of type II phosphatidylinositol phosphate kinase by tyrosine phosphorylation in bovine rod outer segments

Type II phosphatidylinositol phosphate kinase (PIPKII) is an enzyme responsible for the synthesis of phosphatidylinositol-4,5-bisphosphate (PI-4,5-P(2)) from phosphatidylinositol-5-phosphate (PI-5-P). In this study, we demonstrate the presence of PIPKII alpha in bovine photoreceptor rod outer segments (ROS) and the involvement of tyrosine phosphorylation in the regulation of its activity. PIPKII activity in bovine ROS was verified by the preferential conversion of synthetic dipalmitoyl PI-5-P to PI-4,5-P(2), lack of effect of phosphatidic acid, inhibition by heparin, immunoreaction with an anti-PIPKII alpha antibody on Western blots, and immunocytochemical localization in bovine and rat ROS by anti-PIPKII alpha. Immunoprecipitates of bovine ROS with the anti-PIPKII alpha antibody possessed PIPK enzymatic activity and preferentially used PI-5-P as substrate for PI-4,5-P(2) biosynthesis. The activity of PIPKII was greatly increased under conditions favoring tyrosine phosphorylation in ROS, and PIPKII activity was immunoprecipitated with anti-phosphotyrosine (anti-PY) antibodies from tyrosine phosphorylated ROS. Preincubation of ROS with tyrosine kinase inhibitors almost abolished the kinase activity in the anti-PY immunoprecipitates. Immunoblot analysis showed that PIPKII alpha was present in anti-PY immunoprecipitates from phosphorylated ROS but not from nonphosphorylated controls. We conclude that PIPKII alpha is present in ROS and that its activity is regulated by tyrosine phosphorylation.

Biosynthesis of docosahexaenoate-containing glycerolipid molecular species in the retina

Vertebrate retinas are highly enriched in long-chain polyunsaturated fatty acids (PUFA), especially docosahexaenoic acid (22:6n-3, DHA). In the present study, we investigated the role of de novo synthesis in the enrichment of 22:6n-3 in characteristic molecular species of retinal glycerolipids. Following the incubation of fresh dark-adapted retinas with [2-(3)H]-glycerol, individual glycerolipids were isolated and converted into either diacylglycerol acetates (DGAC) or diacylglycerol benzoates (DGBZ), followed by high-performance liquid chromatography (HPLC) and flow-through radioactivity detection. Total lipids from rat retinas incubated with [3H]-glycerol were analyzed. Unlike what was observed with frog retinas, relative larger of amounts of di-22:6 molecular species were synthesized de novo. In both rat and frog retinas, there was synthesis of glycerolipid molecular species containing two PUFA (one of which was 22:6) in larger amounts than predicted by their steady-state mass levels. These results demonstrate that the unique molecular species of retinal glycerolipids are derived only in part through de novo synthesis, but that molecular rearrangement (remodeling) and differential turnover must also play a role in maintaining the high levels of 22:6 found in rod phohtoreceptor outer segments (ROS) membranes.

Brain uptake and utilization of fatty acids: applications to peroxisomal biogenesis diseases

The brain is rich in diverse fatty acids saturated, monounsaturated and polyunsaturated fatty acids with chain lengths ranging from less than 16 to more than 24 carbons that make up the complex lipids present in this organ. While some fatty acids are derived from endogenous synthesis, others must come from exogenous sources. The mechanism(s) by which fatty acids enter cells has been the subject of much debate. While some investigators argue for a protein-mediated process, others suggest that simple diffusion is sufficient. In the brain, uptake is further complicated by the presence of the blood-brain barrier. Brain fatty acid homeostasis is disturbed in many human disorders, as typified by the peroxisomal biogenesis diseases. A workshop designed to bring together researchers from varied backgrounds to discuss these issues in an open forum was held in March, 2000. In addition to assessing the current state of knowledge, areas requiring additional investigation were identified and recommendations for future research were made. A brief overview of the invited talks is presented here.

Tyrosine phosphorylation is involved in phosphatidylinositol 3-kinase activation in bovine rod outer segments

PURPOSE

We have previously shown that phosphatidylinositol 3-kinase (PI 3-kinase) activity is present in bovine rod outer segments (ROS). The present study was undertaken to investigate the mechanism of PI 3-kinase activation in these membranes.

METHODS

Tyrosine-phosphorylated ROS (PY-ROS) were obtained by incubating ROS with ATP, MgCl2, and orthovanadate (Na3VO4), a tyrosine phosphatase inhibitor. Non-phosphorylated ROS (N-ROS) were obtained by incubating ROS under the same conditions, but without ATP and orthovanadate. Both were subjected to immunoprecipitation using antibodies against the regulatory p85 (anti-p85) subunit of PI 3-kinase, the catalytic p110 (anti-p110) subunit of PI 3-kinase, or phosphotyrosine (anti-PY). The immunoprecipitates (IPs) were assayed for PI 3-kinase activity. Enzyme assay products were separated by thin-layer chromatography (TLC), deacylated, and identified by high performance liquid chromatography (HPLC).

RESULTS

PI 3-kinase activity in anti-p85 and p110alpha IPs was significantly higher in PY-ROS than in N-ROS. No enzyme activity was recovered in anti-p110beta IPs. PI 3-kinase activity in anti-PY IPs from PY-ROS was six-fold greater than those from N-ROS. Immunoblot analysis showed that the amount of p85 in PY IPs from PY-ROS was significantly higher than those from N-ROS. However, tyrosine phosphorylation of p85 and p110alpha was not observed in anti-p85 and anti-p110alpha IPs that were probed with anti-PY.

CONCLUSIONS

This study indicates that the p85/p110alpha complex of PI 3-kinase is present in ROS and tyrosine phosphorylation is involved in the regulation of its activity.

Gender differences and the effects of synthetic exogenous and non-synthetic estrogens in focal cerebral ischemia

The role of gender difference and estrogen in ischemic cerebrovascular events is controversial. Evidence is lacking as to whether or not there are significant gender differences in the incidence and outcome of stroke in the clinical setting. Recent clinical epidemiological studies have demonstrated that there is no significant association between the use of hormonal replacement therapy and the risk of stroke. However, several animal studies have shown that there are gender differences in stroke outcome and that exogenous administered estrogens are neuroprotective. In this study, the influence of gender differences and the effects of synthetic and non-synthetic estrogens were examined in a model of focal cerebral ischemia using 210 male, intact female, and ovariectomized female rats. All animals underwent 3 h of middle cerebral artery and bilateral common carotid artery occlusion. After 72 h, the rats were sacrificed and stained for histological assessment of infarction. There were no gender differences in infarction volume. Intravenous administration of either low or high dose 17 beta-estradiol or tibolone did not alter infarct volume. Subcutaneous administration of low and high dose 17beta-estradiol using 7-day release pellets did not alter infarct volume. Low dose tibolone using implanted 7-day release pellets did not alter infarct volume. However, high dose tibolone using implanted 7-day release pellets significantly (P<0.05) reduced infarct volume only in ovariectomized female rats. These results demonstrate that estrogen therapy has no effect on infarction volume following severe focal cerebral ischemia.

Effects on the bispectral index during medium-high dose fentanyl induction with or without propofol supplement

BACKGROUND

The search for a drug-independent monitor to determine depth of anaesthesia and hypnosis continues. The bispectral analysis (BIS) of the EEG correlates well with the clinical dose-response of hypnotic drugs during induction, but the effect on BIS of an opiate induction, as for coronary bypass surgery, is not known.

METHODS

Fourteen patients scheduled for elective coronary bypass surgery were studied. BIS was recorded during induction in 7 patients receiving 10 microg/kg fentanyl without any hypnotic agent and in 7 patients receiving 0.5 mg/kg propofol before the fentanyl dose.

RESULTS

The effect of fentanyl was very variable both regarding BIS and clinical response. Five of the 7 patients that received only fentanyl lost their response to verbal command within 8 min. BIS values at loss of response varied between 45 and 94. One patient remained awake with BIS 43. All 7 patients receiving propofol before the fentanyl dose lost their response to verbal command within 5 min. BIS values at the time for loss of response varied between 78 and 98.

CONCLUSION

Loss of response to verbal command when a medium-high dose of fentanyl is used for induction cannot be distinguished from wakefulness with adequate sensitivity by BIS. The current BIS algorithm seems not to accurately reflect the hypnotic effects of fentanyl.

Persistent posttraumatic cerebrospinal fluid leakage

OBJECT

Persistent posttraumatic cerebrospinal fluid (CSF) leakage frequently complicates skull base fractures. Although many CSF leaks will cease without treatment, patients with CSF leaks that persist greater than 24 hours may be at increased risk for meningitis, and many will require surgical intervention. The authors reviewed their 15-year experience with posttraumatic CSF leaks that persisted longer than 24 hours.

METHODS

The authors reviewed the medical records of 51 patients treated between 1984 and 1998 with CSF leaks that persisted for 24 hours or longer after traumatic head injury. In 27 patients (55%) spontaneous resolution of CSF leakage occurred at an average of 5 days posttrauma. In 23 patients (45%) surgery was required to resolve the leakage. Eight patients (16%) with occult CSF leaks presented with recurrent meningitis at an average of 6.5 years posttrauma. Forty-three (84%) patients with CSF leaks sustained a skull fracture, most commonly involving the frontal sinus, whereas parenchymal brain injury or extraaxial hematoma was demonstrated in only 18 patients (35%). Delayed CSF leaks, with an average onset of 13 days posttrauma, were observed in eight patients (16%). Among patients with clinically evident CSF leakage, the frequency of meningitis was 10% with antibiotic prophylaxis, and 21% without antibiotic prophylaxis. Thus, prophylactic antibiotic administration halved the risk of meningitis. A variety of surgical approaches was used, and no significant neurological morbidity occurred. Three (13%) of 23 surgically treated patients required additional surgery to treat continued CSF leakage.

CONCLUSIONS

A significant proportion of patients with CSF leaks that persist greater than 24 hours will require surgical intervention. Prophylactic antibiotic therapy may be effective in this group of patients. Patients with skull base or frontal sinus fractures should be followed to detect the occurrence of delayed leakage. Surgery-related outcome is excellent.

Light-mediated activation of diacylglycerol kinase in rat and bovine rod outer segments

The hydrolysis of phosphatidylinositol 4,5-bisphosphate is regulated by light in retinal rod outer segment (ROS) membranes. We recently reported that the activities of phosphatidylinositol synthetase and phosphatidylinositol 3-kinase are also higher in bleached (light-exposed) ROS (B-ROS). In this study, we investigated the effect of bleaching on diacylglycerol (DAG) kinase (DAG-kinase) activity in bovine and rat ROS membranes prepared from dark-adapted (D-ROS) or bleached (B-ROS) retinas. In bovine ROS, DAG-kinase activity toward endogenous DAG substrate was higher in B-ROS than in D-ROS. Quantification of DAG in both sets of membranes showed that the levels were the same, eliminating the possibility that the greater DAG-kinase activity was due to higher levels of endogenous substrate in B-ROS. DAG-kinase activity was also higher in B-ROS against an exogenous, water-soluable substrate (1, 2-didecanoyl-rac-glycerol), which competed with endogenous DAG substrate and saturated at approximately 2 mM. Immunoblot analysis with an anti-DAG-kinase gamma polyclonal antibody demonstrated that the gamma isoform was present in isolated bovine ROS. Immunocytochemistry of frozen bovine retinal sections confirmed the presence of DAG-kinase gamma immunoreactivity in ROS, as well as other retinal cells. Quantification of the immunoreactive products on western blots showed that more DAG-kinase gamma was present in B-ROS than in D-ROS. In an in vivo experiment, ROS prepared from rats exposed to 30 min of room light had greater DAG-kinase activity than ROS prepared from dark-adapted animals. Taken together, these data suggest that light exposure leads to the translocation of DAG-kinase from the cytosol to ROS membranes and that the greater DAG-kinase activity in B-ROS is due to the presence of more protein associated with ROS membranes.

Fentanyl and midazolam anaesthesia for coronary bypass surgery: a clinical study of bispectral electroencephalogram analysis, drug concentrations and recall

Bispectral index (BIS) was assessed as a monitor of depth of anaesthesia during fentanyl and midazolam anaesthesia for coronary bypass surgery. In 10 patients given morphine premedication, anaesthesia was induced with a combination of midazolam and fentanyl and thereafter maintained with a continuous infusion of a mixture of midazolam and fentanyl 5 and 50 micrograms kg-1 h-1, respectively. BIS was recorded continuously but not shown to the attending anaesthetist. Plasma concentrations of midazolam and fentanyl were measured five times during the procedure. An auditory stimulus was given during bypass. All patients were interviewed twice after operation for explicit and implicit recall. No patient had any anaesthetic complications. BIS decreased during anaesthesia, but varied considerably during surgery (range 36-91) with eight patients having values > 60. Midazolam and fentanyl drug concentrations did not correlate with BIS. No patient reported explicit or implicit recall. During clinically adequate anaesthesia with midazolam and fentanyl BIS varies considerably. The most likely reason is that BIS is not an accurate measure of the depth of anaesthesia when using this combination of agents.