The overall pattern of cardiac contraction depends on a spatial gradient of myosin regulatory light chain phosphorylation

Evolution of the human heart has incorporated a variety of successful strategies for motion used throughout the animal kingdom. One such strategy is to add the efficiency of torsion to compression so that blood is wrung, as well as pumped, out of the heart. Models of cardiac torsion have assumed uniform contractile properties of muscle fibers throughout the heart. Here, we show how a spatial gradient of myosin light chain phosphorylation across the heart facilitates torsion by inversely altering tension production and the stretch activation response. To demonstrate the importance of cardiac light chain phosphorylation, we cloned a myosin light chain kinase from a human heart and have identified a gain-in-function mutation in two individuals with cardiac hypertrophy.

The stretch-activation response may be critical to the proper functioning of the mammalian heart

The "stretch-activation" response is essential to the generation of the oscillatory power required for the beating of insect wings. It has been conjectured but not previously shown that a stretch-activation response contributes to the performance of a beating heart. Here, we generated transgenic mice that express a human mutant myosin essential light chain derived from a family with an inherited cardiac hypertrophy. These mice faithfully replicate the cardiac disease of the patients with this mutant allele. They provide the opportunity to study the stretch-activation response before the hearts are distorted by the hypertrophic process. Studies disclose a mismatch between the physiologic heart rate and resonant frequency of the cardiac papillary muscles expressing the mutant essential light chain. This discordance reduces oscillatory power at frequencies that correspond to physiologic heart-rates and is followed by subsequent hypertrophy. It appears, therefore, that the stretch-activation response, first described in insect flight muscle, may play a role in the mammalian heart, and its further study may suggest a new way to modulate human cardiac function.

Acute gouty sacroiliitis: a case report and review of the literature

A 45-year-old black woman with an acute attack of severe unilateral sacroiliitis, negative microbiologic studies, is presented. An open biopsy specimen of the sacroiliac joint revealed evidence of tophaceous gout. The unique features of the case, differential diagnosis, and pertinent literature on gouty sacroiliitis are reviewed. Acute gouty sacroiliitis, although it is rare, does occur and should be included in the differential diagnosis of unilateral sacroiliitis in patients with a long-standing history of gout.

Familial occurrence of hypersensitivity to phenytoin

PURPOSE

Therapy with anticonvulsants such as phenytoin, phenobarbital, and carbamazepine can be complicated by severe hypersensitivity reactions. Previous work has suggested that the predisposition to such reactions is based on an inherited abnormality in the detoxification of reactive metabolites of the drugs. However, there are no reports of familial occurrence of the reactions in the literature. In the current study, we examined a family in which three siblings developed hypersensitivity reactions to phenytoin, confirming the inheritance of a predisposition to the reactions. Detoxification of reactive metabolites of the anticonvulsants was studied in cells from the patients and their siblings.

PATIENTS AND METHODS

Three siblings from a family of 12 siblings developed hypersensitivity reactions to phenytoin characterized by fever, rash, lymphadenopathy, and anicteric hepatitis. All recovered completely after discontinuation of treatment. One sibling tolerated phenobarbital without toxic sequelae. Peripheral blood mononuclear cells from the three patients and five additional siblings who had never taken anticonvulsants were exposed to oxidative metabolites of phenytoin, phenobarbital, and carbamazepine generated by a hepatic microsomal drug-metabolizing system in vitro. The toxicity of metabolites in the cells from the siblings was compared with that in cells from control subjects.

RESULTS

Cells from each of the patients who had experienced a hypersensitivity reaction exhibited increased toxicity from metabolites of phenytoin and carbamazepine, while the cellular response to metabolites of phenobarbital was within normal limits. Cells from four of the other siblings showed an abnormal response to phenytoin metabolites, while cells from the final sibling detoxified phenytoin metabolites normally.

CONCLUSION

Our observations on the patients confirm the inherited nature of phenytoin hypersensitivity reactions in vivo. In vitro studies demonstrated abnormal metabolite detoxification in the patients and several of their siblings. The detoxification defect included metabolites of phenytoin and carbamazepine but not of phenobarbital. A family history of a drug hypersensitivity reaction should alert physicians to the probability of a markedly increased risk of an adverse reaction in family members. In vitro assays to confirm adverse reaction risks may ultimately be able to provide individualized risk assessment for patients who must take anticonvulsants.

Effect of temperature on sodium-calcium exchange in sarcolemma from mammalian and amphibian hearts

We have investigated temperature dependence of Ca2+ uptake by the cardiac sarcolemmal Na(+)-Ca2+ exchanger from dog, rabbit and bullfrog. In native rabbit sarcolemmal vesicles, Ca2+ affinity of the Na(+)-Ca2+ exchanger is unchanged from 7 to 37 degrees C; however, the initial velocity of Ca2+ uptake declines much more steeply below 22 degrees C than above 22 degrees C. In native dog sarcolemma, the temperature dependence of Na(+)-Ca2+ exchange velocity is similar to that of native rabbit. However, in frog heart the velocity of Na(+)-Ca2+ exchange declines much more slowly with decreasing temperature at both temperature ranges. Reconstitution of the Na(+)-Ca2+ exchanger into artificial lipid vesicles consisting of either asolectin or phosphatidylserine, phosphatidylcholine, and cholesterol has little effect on temperature dependence of Na(+)-Ca2+ exchange velocity in any of the three species. We conclude that the lesser temperature sensitivity of the cardiac sarcolemmal Na(+)-Ca2+ exchanger of a poikilothermic species is at least partly an intrinsic property of the transport protein.

Identification of the cardiac sarcolemmal Na(+)-Ca2+ exchanger using monoclonal antibodies

We have previously partially purified the sarcolemmal Na(+)-Ca2+ exchange protein and produced rabbit polyclonal antibodies to the exchanger (Philipson, K.D., Longoni, S., Ward, R. 1988. Biochim. Biophys. Acta 945:298-306). We now describe the generation of three stable murine hybridoma lines which secrete monoclonal antibodies (MAb's) to the exchanger. These MAb's immunoprecipitate 50-75% of solubilized Na(+)-Ca2+ exchange activity. The MAb's appear to be reactive with native conformation-dependent epitopes on the Na(+)-Ca2+ exchanger since they do not react on immunoblots. An indirect method was used to identify Na(+)-Ca2+ exchange proteins. A column containing Na(+)-Ca2+ exchanger immobilized by MAb's was used to affinity purify the rabbit polyclonal antibody. The affinity-purified polyclonal antibody reacted with proteins of apparent molecular weights of 70, 120, and 160 kDa on immunoblots of sarcolemma. The data provide strong support for our previous association of Na(+)-Ca2+ exchange with these proteins.

Influence of phospholipid fatty acyl composition on sarcolemmal and sarcoplasmic reticular cation transporters

Using solubilization/reconstitution techniques, we have investigated the influence of membrane fatty acyl composition on the activities of sarcolemmal and sarcoplasmic reticular transporters. The sarcolemmal Na(+)-Ca2+ exchanger and Na+, K(+)-ATPase and the sarcoplasmic reticular Ca2(+)-ATPase were reconstituted into phosphatidylcholine:phosphatidylserine:cholesterol (30:50:20% by weight) proteoliposomes of defined fatty acyl composition. Transport activities varied considerably with phospholipid fatty acyl composition. Quite strikingly, the dependence on membrane fatty acyl composition for all three transporters was identical.

Ouabain treatment of cardiac cells induces enhanced Na+-Ca2+ exchange activity

Inhibition of the cardiac Na+-K+-ATPase with cardiac glycosides causes a rise in internal Na+ and a subsequent increase in cellular Ca2+ due to the sarcolemmal Na+-Ca2+ exchange mechanism. We investigated the adaptation of cultured cardiac cells to prolonged elevation of internal Ca2+ after exposure to ouabain. Cultured neonatal rat heart cells were treated with 100 microM ouabain for 4-48 h. This ouabain concentration inhibited Na+-K+-ATPase activity by approximately 45% and caused modest cellular Ca2+ loading. We found that cells adapted to ouabain treatment by increasing the amount of sarcolemmal Na+-Ca2+ exchange activity by 50-90% over a 24-h period. Kinetic and immunological data indicate that the increase was due to increased numbers of functional exchangers. Neither total cellular nor total sarcolemmal protein content was affected by the ouabain treatment. These results may be relevant toward understanding the effects of therapeutic use of cardiac glycosides.

Influence of sterols and phospholipids on sarcolemmal and sarcoplasmic reticular cation transporters

We have examined the influence of different sterols and phospholipids on the activities of the cardiac sarcolemmal Na+-Ca2+ exchanger and Na+,K+-ATPase and the sarcoplasmic reticular Ca2+-ATPase in reconstituted proteoliposomes. When either the solubilized Na+-Ca2+ exchanger or the Na+,K+-ATPase is reconstituted into phosphatidylcholine (PC):phosphatidylserine (30:50 by weight) vesicles, high cholesterol levels (20% by weight) are required for activity to be expressed. This sterol requirement is highly specific for cholesterol. Several cholesterol analogues with minor structural changes are unable to support Na+-Ca2+ exchange or Na+,K+-ATPase activities. When solubilized sarcolemma is reconstituted into PC:cardiolipin vesicles, however, the requirement for cholesterol is lost. Substantial activity can be obtained in the complete absence of cholesterol or in the presence of several cholesterol analogues. Thus, sterol/protein interactions can be highly dependent on the phospholipid environment. In contrast, the skeletal muscle sarcoplasmic reticular Ca2+-ATPase functions equally well in the presence or absence of cholesterol after reconstitution into either PC:phosphatidylserine or PC:cardiolipin proteoliposomes. Phospholipid requirements of the transporters were also examined. The sarcolemmal Na+-Ca2+ exchanger, Na+,K+-ATPase, and the sarcoplasmic reticular Ca2+-ATPase all function optimally in the presence of phosphatidylserine or cardiolipin after reconstitution. Thus, the sarcolemmal cation transporters have similar sterol and phospholipid requirements and may have structural similarities in their hydrophobic regions. The sarcoplasmic reticular Ca2+ pump evolved in a low cholesterol membrane and has different lipid interactions. These findings may have general applicability to other plasma membrane and endoplasmic reticular enzymes.

Studies on oxygen and extracellular fluid restrictions in cultured heart cells: high energy phosphate metabolism

Although cultured heart cells are increasingly used for the study of cardiac metabolism, relatively little is known about their energy turnover. We studied the effects of anoxia with simultaneous restrictions of the volume of the extracellular medium ("ischaemia") on high energy phosphate catabolism in cells from neonatal rat ventricles, cultured for 5 days. The cells were incubated for up to 4 h in Ham-F10 medium either in the presence or in the absence of glucose. High energy phosphates in cell extracts and AMP catabolites in the incubation medium were measured by high pressure liquid chromatography. ATP and creatine phosphate content in normoxic cells did not change significantly, either in the presence or absence of glucose, and the values were similar to those found in the heart in vivo. Energy rich phosphates decreased during anoxia, and were more rapidly depleted during simultaneous oxygen deprivation and volume restriction. Glucose delayed the decline in high energy phosphates. In the presence of glucose, hypoxanthine uptake was higher during normoxia than in anoxia, whereas in "ischaemic" conditions some hypoxanthine was produced. In the absence of glucose, only minor changes were observed in hypoxanthine levels during anoxia, but hypoxanthine production was marked when anoxia was coupled with extracellular volume restriction. Adenosine levels were below the limit of detection. Inosine release was relatively low under all conditions, Xanthine release did not show variation, and anoxia suppressed urate production. Oxygen and glucose deprivation thus led to various degrees of ATP and creatine phosphate breakdown in cultured neonatal heart cells both during anoxia and in simulated "ischaemia".

Protein methylation inhibits Na+-Ca2+ exchange activity in cardiac sarcolemmal vesicles

We have examined the effect of membrane methylation on the Na+-Ca2+ exchange activity of canine cardiac sarcolemmal vesicles using S-adenosyl-L-methionine as methyl donor. Methylation leads to approximately 40% inhibition of the initial rate of Nai+-dependent Ca2+ uptake. The inhibition is due to a lowering of the Vmax for the reaction. The inhibition is not due to an effect on membrane permeability and is blocked by S-adenosyl-L-homocysteine, an inhibitor of methylation reactions. The following experiments indicated that inhibition of Na+-Ca2+ exchange was due to methylation of membrane protein and not due to methylated phosphatidylethanolamine (PE) compounds (i.e., phosphatidyl-N-monomethylethanolamine (PMME) or phosphatidyl-N,N'-dimethylethanolamine (PDME]: (1) We solubilized sarcolemma and reconstituted activity into vesicles containing no PE. The inhibition by S-adenosyl-L-methionine was not diminished in this environment. (2) We reconstituted sarcolemma into vesicles containing PMME or PDME. These methylated lipid components had no effect on Na+-Ca2+ exchange activity. (3) We verified that many membrane proteins, probably including the exchanger, become methylated.

Phospholipid composition modulates the Na+-Ca2+ exchange activity of cardiac sarcolemma in reconstituted vesicles

Na+-Ca2+ exchange activity in cardiac sarcolemmal vesicles is known to be sensitive to charged, membrane lipid components. To examine the interactions between membrane components and the exchanger in more detail, we have solubilized and reconstituted the Na+-Ca2+ exchanger into membranes of defined lipid composition. Our results indicate that optimal Na+-Ca2+ exchange activity requires the presence of certain anionic phospholipids. In particular, phosphatidylserine (PS), cardiolipin, or phosphatidic acid at 50% by weight results in high Na+-Ca2+ exchange activity, whereas phosphatidylinositol and phosphatidylglycerol provide a poor environment for exchange. In addition, incorporation of cholesterol at 20% by weight greatly facilitates Na+-Ca2+ exchange activity. Thus, for example, an optimal lipid environment for Na+-Ca2+ exchange is phosphatidylcholine (PC, 30%)/PS (50%)/cholesterol (20%). Na+-Ca2+ exchange activity is also high when cardiac sarcolemma is solubilized and then reconstituted into asolectin liposomes. We fractionated the lipids of asolectin into subclasses for further reconstitution studies. When sarcolemma is reconstituted into vesicles formed from the phospholipid component of asolectin, Na+-Ca2+ exchange activity is low. When the neutral lipid fraction of asolectin (including sterols) is also included in the reconstitution medium, Na+-Ca2+ exchange activity is greatly stimulated. This result is consistent with the requirement for cholesterol described above. Proteinase treatment, high pH, intravesicular Ca2+ and dodecyl sulfate all stimulate Na+-Ca2+ exchange in native sarcolemmal vesicles. We examined the effects of these interventions on exchange activity in reconstituted vesicles of varying lipid composition. In general, Na+-Ca2+ exchange could be stimulated only when reconstituted into vesicles of a suboptimal lipid composition. That is, when reconstituted into asolectin or PC/PS/cholesterol (30:50:20), the exchanger is already in an activated state and can no longer be stimulated. The one exception was that the Na+-Ca2+ exchanger responded to altered pH in an identical manner, independent of vesicle lipid composition. The mechanism of action of altered pH on the exchanger thus appears to be different from other interventions.

Studies on oxygen and volume restriction in cultured cardiac cell: possible rearrangement of sarcolemmal lipid moieties during anoxia and ischemia-like states

Cultured heart cells have been shown useful for investigating states of oxygen and volume restrictions, simulating anoxia and ischemia-like states at cellular levels. The sarcolemma has been implicated as one of the early sites of ischemic damage; therefore, lactoperoxidase catalyzed radioiodination was used to study accessibility of the sarcolemmal lipid moieties to this enzymatic probe, reflecting their exposure to the extracellular environment, hence the biophysical state of the sarcolemma. These studies have shown that within one hour of 'ischemic' injuries: (1) The degree of labelling in the total phospholipid fraction is considerably increased; and (2) Profound changes in the relative extent of labelling of different phospholipid classes were observed. The PE/PC labelling ratio increases dramatically with the progress of ischemia-like state. We suggest that early during ischemic injury, reorganization of the cell surface phospholipids occurs and discuss possible relations to the energy charge of the cell.

Studies on oxygen and volume restrictions in cultured cardiac cells. I. A model for ischemia and anoxia with a new approach

A novel incubation unit is described that is highly suitable for thorough studies of oxygen deprivation states. Its application with cultured heart cells is experimentally demonstrated. The release of enzymes, taken as a marker for cell damage, has clearly shown that restriction of the volume of extracellular medium combined with oxygen plus glucose deprivation caused greatest cellular damage. It may be considered as an experimental ischemia-like state. Furthermore, the onset of cellular damage followed a time table very much like that occurring in vivo under similar conditions, more so than any other previously described studies. A time lag between the release of cytoplasmic enzymes and lysosomal enzymes and other observations made in the present study suggests a sequential order of events in which the release of cytoplasmic enzymes occurs at a stage of reversible damage due to oxygen deprivation, whereas the release of lysosomal enzymes may point at irreparable damage.