Biomedical subjects

Lawrence D Gaspers

Publications and source records attributed to Lawrence D Gaspers.

6 recordsLinked to original sources

PubMed2006

Ryanodine receptors in liver.

The ryanodine receptor has been mainly regarded as the Ca2+ release channel from sarcoplasmic reticulum controlling skeletal and cardiac muscle contraction. However, many studies have shown that it is widely expressed, with functions not restricted to muscular contraction. This study examined whether ryanodine receptor plays a role in calcium signaling in the liver. RT-PCR analysis of isolated hepatocytes showed expression of a truncated type 1 ryanodine receptor, but no type 2 or type 3 message was detected. We also detected binding sites for [3H]ryanodine in the microsomal cellular fraction and in permeabilized hepatocytes. This binding was displaced by caffeine and dantrolene, but not by ruthenium red, heparin or cyclic ADP-Ribose. Ryanodine, by itself, did not trigger Ca2+ oscillations in either primary cultured hepatocytes or hepatocytes within the intact perfused rat liver. In both preparations, however, ryanodine significantly increased the frequency of the cytosolic free [Ca2+] oscillations evoked by an alpha1 adrenergic receptor agonist. Experiments in permeabilized hepatocytes showed that both ryanodine and cyclic ADP-ribose evoked a slow Ca2+ leak from intracellular stores and were able to increase the Ca2+-released response to a subthreshold dose of inositol 1,4,5-trisphosphate. Our findings suggest the presence of a novel truncated form of the type 1 ryanodine receptor in rat hepatocytes. Ryanodine modulates the pattern of cytosolic free [Ca2+] oscillations by increasing oscillation frequency. We propose that the Ca2+ released from ryanodine receptors on the endoplasmic reticulum provides an increased pool of Ca2+ for positive feedback on inositol 1,4,5-trisphosphate receptors.

Animals↗

PubMed2006

Models of IP3 and Ca2+ oscillations: frequency encoding and identification of underlying feedbacks.

Hormones that act through the calcium-releasing messenger, inositol 1,4,5-trisphosphate (IP3), cause intracellular calcium oscillations, which have been ascribed to calcium feedbacks on the IP3 receptor. Recent studies have shown that IP3 levels oscillate together with the cytoplasmic calcium concentration. To investigate the functional significance of this phenomenon, we have developed mathematical models of the interaction of both second messengers. The models account for both positive and negative feedbacks of calcium on IP3 metabolism, mediated by calcium activation of phospholipase C and IP3 3-kinase, respectively. The coupled IP3 and calcium oscillations have a greatly expanded frequency range compared to calcium fluctuations obtained with clamped IP3. Therefore the feedbacks can be physiologically important in supporting the efficient frequency encoding of hormone concentration observed in many cell types. This action of the feedbacks depends on the turnover rate of IP3. To shape the oscillations, positive feedback requires fast IP3 turnover, whereas negative feedback requires slow IP3 turnover. The ectopic expression of an IP3 binding protein has been used to decrease the rate of IP3 turnover experimentally, resulting in a dose-dependent slowing and eventual quenching of the Ca2+ oscillations. These results are consistent with a model based on positive feedback of Ca2+ on IP3 production.

Animals↗

PubMed2004

Trauma-hemorrhagic shock mesenteric lymph induces endothelial apoptosis that involves both caspase-dependent and caspase-independent mechanisms.

OBJECTIVE: To determine the mechanism by which gut-derived factors present in mesenteric lymph from rats subjected to trauma-hemorrhagic shock (T/HS) induce endothelial cell death. SUMMARY BACKGROUND DATA: Intestinal ischemia after hemorrhagic shock results in gut barrier dysfunction and the subsequent production of biologically active and tissue injurious factors by the ischemic gut. These factors are carried in the mesenteric lymph and reach the systemic circulation via the mesenteric lymph, thereby ultimately resulting in distant organ injury. Although studies have established that trauma-hemorrhagic (T/HS) shock but not trauma-sham-shock (T/SS) mesenteric lymph is cytotoxic to endothelial cells, whether T/HS lymph-induced endothelial cell death occurs via an apoptotic or a necrotic pathway is unknown. The mechanisms underlying T/HS lymph-induced cytotoxicity are likewise unknown. METHODS: Human umbilical vein endothelial cell (HUVEC) monolayers were incubated with medium, sham-shock, or post shock mesenteric lymph (5%) for 4 hours, after which the mode of cell death (ie, apoptosis versus necrosis) was determined using morphologic (confocal microscopy), biochemical (nucleosomal release), and DNA-based (gel electrophoresis) assays. To clarify the cellular pathways involved in T/HS lymph-induced HUVEC cell death, caspase-3, caspase-9, caspase-8, and BID activity was measured as was the ability of the pan-caspase inhibitor z-VAD-fmk to prevent T/HS lymph-induced cell death. RESULTS: T/HS, but not T/SS, mesenteric lymph or medium was cytotoxic and caused the appearance of the classic morphologic signs of apoptosis, including membrane blebbing, cell shrinkage, and apoptotic body formation. Nucleosomal release and a DNA laddering pattern was also observed in the HUVECs incubated with T/HS lymph. These signs of apoptosis were associated with increased caspase activity as reflected in activation of the pro-apoptotic caspases, caspase-8, -9, and -3, as well as the pro-apoptotic bcl-2-related protein BID. However, since the broad-spectrum caspase inhibitor z-VAD-fmk delayed T/HS lymph-induced HUVEC cell death, but did not prevent it fully, it appears that other factors besides caspases are involved in the endothelial cell toxicity of T/HS lymph. CONCLUSIONS: Gut-derived factors in T/HS, but not T/SS, mesenteric lymph cause endothelial cell death via an apoptotic mechanism that involves both caspase-dependent and caspase-independent pathways.

Animals↗

PubMed2002

Inducible nitric-oxide synthase attenuates vasopressin-dependent Ca2+ signaling in rat hepatocytes.

Increases in both Ca(2+) and nitric oxide levels are vital for a variety of cellular processes; however, the interaction between these two crucial messengers is not fully understood. Here, we demonstrate that expression of inducible nitric-oxide synthase in hepatocytes, in response to inflammatory mediators, dramatically attenuates Ca(2+) signaling by the inositol 1,4,5-trisphosphate-forming hormone, vasopressin. The inhibitory effects of induction were reversed by nitric oxide inhibitors and mimicked by prolonged cyclic GMP elevation. Induction was without effect on Ca(2+) signals in response to AlF(4)(-) or inositol 1,4,5-trisphosphate, indicating that phospholipase C activation and release of Ca(2+) from inositol 1,4,5-trisphosphate-sensitive Ca(2+) stores were not targets for nitric oxide inhibition. Vasopressin receptor levels, however, were dramatically reduced in induced cultures. Our data provide a possible mechanism for hepatocyte dysfunction during chronic inflammation.

Animals↗

PubMed2002

Inhibition of the mitochondrial permeability transition by aldehydes.

Fructose has been shown to protect hepatocyte viability during hypoxia or exposure to mitochondrial electron transport inhibitors. We report here that the fructose metabolite D-glyceraldehyde (D-GA) is a good inhibitor of the mitochondrial permeability transition pore (PTP) in isolated rat liver mitochondria. We propose that a substantial portion of the protective effect of fructose on hepatocytes is due to D-GA inhibition of the permeability transition. Aldehydes which are substrates of the mitochondrial aldehyde dehydrogenase (mALDH) afford protection, while poor substrates do not. Protection is prevented by the ALDH inhibitor chloral hydrate. We propose that the NADH/NAD(+) ratio is the key to protection. The aldehydes phenylglyoxal (PGO) and 4-hydroxynonenal (4-HNE), which have previously been shown to inhibit the PTP, apparently function by a different mechanism independent of mALDH activity. Both PGO or 4-HNE are themselves potent inhibitors of ALDH, and their protective effect cannot be blocked by an ALDH inhibitor.

Aldehyde Dehydrogenase↗

PubMedDate not supplied

Calcium signaling in liver.

In hepatocytes, hormones linked to the formation of the second messenger inositol 1,4,5-trisphosphate (InsP3) evoke transient increases or spikes in cytosolic free calcium ([Ca2+]i), that increase in frequency with the agonist concentration. These oscillatory Ca2+ signals are thought to transmit the information encoded in the extracellular stimulus to down-stream Ca2+-sensitive metabolic processes. We have utilized both confocal and wide field fluorescence microscopy techniques to study the InsP3-dependent signaling pathway at the cellular and subcellular levels in the intact perfused liver. Typically InsP3-dependent [Ca2+]i spikes manifest as Ca2+ waves that propagate throughout the entire cytoplasm and nucleus, and in the intact liver these [Ca2+]i increases are conveyed through gap junctions to encompass entire lobular units. The translobular movement of Ca2+ provides a means to coordinate the function of metabolic zones of the lobule and thus, liver function. In this article, we describe the characteristics of agonist-evoked [Ca2+]i signals in the liver and discuss possible mechanisms to explain the propagation of intercellular Ca2+ waves in the intact organ.

Animals↗