Biosynthesis and secretion of proatrial natriuretic factor by cultured rat cardiocytes.

Rat atrial natriuretic factor (ANF) is translated as a 152-amino acid precursor preproANF. PreproANF is converted to the 126-amino acid proANF, the storage form of ANF in the atria. ANF isolated from the blood is approximately 25 amino acids long. It is demonstrated here that rat cardiocytes in culture store and secrete proANF. Incubation of proANF with serum produced a smaller ANF peptide. PreproANF seems to be processed to proANF in the atria, and proANF appears to be released into the blood, where it is converted by a protease to a smaller peptide.

Cellular processing of pro-atrial natriuretic factor (pro-ANF): studies using an antiserum that selectively binds ANF-(99-126) after its cleavage from pro-ANF.

Atrial natriuretic factor (ANF) is stored in atrial myocytes as a 15-17K prohormone, but circulates in plasma as a 3K, carboxy (C)-terminal fragment of the prohormone. The tissue location at which the cleavage of pro-ANF to its hormonal form occurs is unknown. In the present study, an immunological approach was taken to address this question. A polyclonal antiserum was generated which recognizes the hormonal form of ANF [ANF-(99-126)] only after its cleavage from the prohormone. This was accomplished by immunizing rabbits with a synthetic peptide corresponding to the seven amino (N)-terminal residues of ANF-(99-126) coupled to carrier protein via a C-terminal cysteine. This antiserum, anti-ANF-(99-105), demonstrated high affinity for ANF-(99-126) (IC50 = 170 pM), but displayed 100-fold less affinity for recombinant pro-ANF [ANF-(2-126)]. The N-terminal specificity of anti-ANF-(99-105) was evident by its failure to bind ANF-(103-126) at concentrations up to 100 nM. The specificity of anti-ANF-(99-105) for the hormonal form of ANF was examined by using thrombin to cleave pro-ANF and testing for the generation of anti-ANF-(99-105) immunoreactivity. Cleavage of atrial pro-ANF or 35S biosynthetically-labeled pro-ANF resulted in the production of immunoreactive material from the prohormone, whereas pro-ANF itself demonstrated no cross-reactivity with anti-ANF-(99-105). Anti-ANF-(99-105) could also recognize ANF released from the isolated perfused rat heart. When anti-ANF-(99-105) was used in immunohistochemical studies of rat atrial myocardium, no staining was observed in unfixed frozen sections. This suggests that proteolytic processing of pro-ANF is not an intracardiocytic event.

Molecular studies of the atrial natriuretic factor gene.

The molecular biology of human atrial natriuretic factor was studied. A cloned rat cDNA probe was used to analyze tissue for the synthesis of atrial natriuretic factor, and the human gene was identified and sequenced. Nucleotide sequence comparison of human and rodent atrial natriuretic factor genes suggests regions that are critical for regulated expression of this cardiac hormone.

Apneic oxygenation in apnea tests for brain death. A controlled trial.

We performed a prospective controlled study of apneic oxygenation on 15 patients undergoing apnea tests for brain death. All patients were preoxygenated with 100% oxygen at existing respirator settings. During the 10-minute apnea tests, nine patients were given continuous apneic oxygenation by tracheal cannula. The other six patients had tracheal tubes open to room air. The patients given apneic oxygenation had little or no hypoxia by the end of the test. The patients given room air during the test became hypoxic. Many neurologists perform apnea tests with no oxygenation or with preoxygenation alone. This is the first prospective controlled study (to our knowledge) of apneic oxygenation; it shows that preoxygenation alone does not prevent hypoxia during apnea tests for brain death. We recommend that all apnea tests be performed with apneic oxygenation.

Isaacs' syndrome with muscle hypertrophy reversed by phenytoin therapy.

A 16-year-old boy was seen for severe episodic muscle cramps and generalized myokymia, consistent with Isaacs's syndrome. Bilateral calf hypertrophy (46-cm calf circumference) and ankle areflexia were noted. He was treated with phenytoin sodium, 300 mg/day. Within three months there was marked decrease in myokymia, total relief of cramps, return of ankle reflexes, and 6-cm reduction in calf circumference. We suggest that the excess muscle activity in Isaacs's syndrome may be responsible for the associated phenomena of muscle hypertrophy and areflexia.

Specificity of serine proteases for cleavage sites on proatrial natriuretic factor.

An immunological approach was used to investigate the specificity of protease cleavage sites on proANF. Cleavage of 35S-cysteine biosynthetically-labeled proANF by whole serum, thrombin and kallikrein was examined. Reaction products were immunoprecipitated with two antibodies directed to different epitopes: a previously characterized antibody directed toward the carboxy-terminus of ANF103-126, which cross reacts with proANF, ANF99-126 and ANF103-126, and a newly prepared antisera to synthetic ANF99-105, which uniquely recognizes ANF99-126, but not proANF or ANF103-126. With increasing time of incubation with rat serum, proANF is sequentially cleaved at the C-terminus of a monobasic Pro-Arg dipeptide sequence to form ANF99-126, and then at the C-terminus of a dibasic Arg-Arg dipeptide sequence to yield ANF103-126. This cleavage activity of serum is blocked by leupeptin (40 micrograms/ml), but not by hirudin (100 nM), a specific inhibitor of thrombin, or by aprotinin (200 KIU/ml), a kallikrein inhibitor. When 100-fold purified serum cleavage enzyme was used in place of crude serum, similar results were obtained. Thrombin cleaves proANF only at the monobasic site to produce ANF99-126 while kallikrein cleaves only at the dibasic site to produce ANF103-126. As expected, the generation of these cleavage products can be inhibited by hirudin or aprotinin respectively. These data indicate that the substrate specificity of the serum cleavage activity is broader than that of thrombin or kallikrein, and that cleavage of proANF by serum proteases may be influenced by conformational restraints. The methods developed here should help in the future characterization of the physiological proANF cleaving enzyme.

Analysis of atrial natriuretic factor biosynthesis and secretion in adult and neonatal rat atrial cardiocytes.

Atrial natriuretic factor (ANF) is stored in atrial cardiocytes as the 126 amino acid polypeptide, proANF, which is later cleaved to the 24-28 amino acid carboxyterminal peptides, the major circulating forms. Earlier studies have demonstrated that isolated, cultured neonatal rat cardiocytes both store and secrete proANF, which can be cleaved to the smaller circulating form(s) by a serum protease. Since differences may exist between neonatal and adult cardiocytes with respect to ANF synthesis and processing, we compared the forms of ANF stored and secreted by neonatal rat cardiocytes with those of adult cells. Using four to five day cultures of isolated atrial cardiocytes prepared from the hearts of neonatal and adult rats, pulse-chase studies were performed with 35S-cysteine and 35S-methionine. Analysis of ANF stored and secreted by these cells was performed by immunoprecipitation of cell extracts and culture media using antibodies directed to either the carboxyterminus or aminoterminus of proANF followed by SDS-PAGE and autoradiography. Cell extracts from both adult and neonatal cultures were found to contain only a 17-kDa polypeptide, previously identified as proANF. The predominant form found in the culture media was also the 17-kDa peptide, with smaller quantities of its 3-kDa carboxyterminal and 14-kDa aminoterminal cleavage products. We conclude from these studies that proANF is the major form stored and secreted by both adult and neonatal cardiocytes in culture; the activity of the protease that cleaves proANF to the smaller forms found in the circulation is either attenuated or is overwhelmed by high ANF-secretory rates in these cultures. Alternatively, the ANF processing and secretory pathways may be somehow altered in culture such that proANF escapes protease cleavage. Further studies will elucidate the nature and location of this protease.

Brain death in perspective.

Advances in lifesaving medical therapy over the past two decades have forced us to redefine death. Cessation of the heartbeat cannot be synonymous with death if the heart can be permanently removed and replaced with a plastic pump. We now can substitute for the function of all organs in the body, save one--the brain. When brain function is irreversibly lost, the person served by that brain has ceased to exist as a person and is medically and legally dead. This article focuses on the criteria currently used to diagnose irreversible brain failure.

Single-dose oral phenytoin loading.

A single 18 mg/kg dose of oral phenytoin capsules or suspension (mean dose, 1.3 g) was given to 44 patients with recent seizures and no detectable serum phenytoin level. Mean serum phenytoin levels after loading for patients receiving capsules were 6.8 micrograms/mL at two hours, 9.7 micrograms/mL at three to five hours, 12.3 micrograms/mL at six to ten hours, and 15.1 micrograms/mL at 16 to 24 hours. Mean levels for patients receiving suspension were slightly, but not significantly, lower than for patients receiving capsules. No seizures occurred during an eight-hour observation period after loading. Drug toxicity was minimal. Single-dose, 18 mg/kg oral phenytoin loading provides rapid therapeutic levels and is well tolerated.

Use of CT in epilepsy: does it matter how long you wait?

Computer tomography (CT) of the brain is of value for finding potentially correctable lesions in adult patients with new onset seizures. The value of CT is unknown, however, for finding such lesions in adult chronic epileptic patients without prior CT. We compared a group of 177 adult patients who had CT within a year from the onset of seizures to a group of 93 patients who had a history of seizures for more than a year before CT was performed. In the first group, 33 potentially correctable lesions (19%) were found including 17 tumors. The group with chronic epilepsy had 4 (4%) potentially correctable lesions: 3 arteriovenous malformations and 1 meningioma. It seems that CT is of value in discovering potentially removable lesions in chronic epileptic patients, but the likelihood is relatively small. The incidence of stable structural lesions seems to be similar in the two groups.

Serum-mediated enhancement of ANF accumulation in the culture medium of cardiac atriocytes.

Cultured cardiac myocytes provide a useful system for investigating ANF biosynthesis and regulation. It has previously been demonstrated that the predominant form of ANF stored in and released from this myocyte model is the 17 kD prohormone, proANF. We report here that as quantitated both by radioimmunoassay and by SDS-PAGE of intrinsically labelled ANF released from these cardiocytes, the addition of serum promotes a 5-6 fold increase in ANF accumulation in the medium of these cells as compared to ANF accumulation in the presence of a defined chemical medium alone. The stimulating effect of serum is immediate and persists in a linear manner for at least 120 minutes. This effect of serum can be reproduced by the addition of albumin or other proteins to the medium but not by alterations in osmolality. Whether this phenomenon represents enhanced release of proANF or is secondary to inhibition of proANF degradation has yet to be determined.

Selective deep peroneal nerve injury associated with arthroscopic knee surgery.

We report 3 cases of isolated deep peroneal nerve injury as a complication of arthroscopic knee surgery. At the level of the knee joint, the deep and superficial peroneal nerves are usually joined as the common peroneal nerve. However, because of the fascicular structure, a partial nerve injury can result in an isolated injury to the deep peroneal nerve fibers. Due to the intraneural topography of the peroneal nerve, electrodiagnostic studies in a partial nerve injury may erroneously indicate a more distal lesion.

Nosocomial stroke.

Nosocomial stroke occurs during hospitalization for unrelated problems. Increased understanding of this relatively ignored entity may provide the key to improved stroke prophylaxis for the hospitalized patient at risk and provide clues to the precipitants of stroke in the general population. We compared nosocomial stroke to stroke occurring outside the hospital in a mixed prospective and retrospective analysis of 372 consecutive strokes occurring over 2 years. We excluded nosocomial stroke associated with cardiac bypass surgery, carotid endarterectomy, and cerebral angiography for cerebrovascular disease because of the known associations of these procedures with stroke. Of our 372 strokes, 47 were nosocomial. There were no significant age and sex differences between nosocomial stroke and stroke admissions. Nosocomial stroke patients were significantly more likely than stroke admission patients to be normotensive (p = 0.001), diabetic (p = 0.01), and have cardiac disease (p = 0.03). Nosocomial stroke patients were significantly less likely to have brain hemorrhages (p = 0.001), lacunar infarcts (p = 0.03), or infarcts of undetermined cause (p = 0.047). Half of the nosocomial stroke patients died versus 11% of stroke admission patients. Nosocomial stroke differs in stroke type, associated diseases, and prognosis from stroke occurring outside the hospital.

Silent myocardial ischemia. Is the person or the event silent?

The symptoms of organic disease vary widely among patients with the same tissue abnormality, because the experience of a symptom is shaped by the patient's perceptual and cognitive style. Thus, the relationship between myocardial ischemia and chest pain is variable in that many patients experience pain without ischemia and many others exhibit ischemia without pain-termed "silent" or "asymptomatic ischemia." Although the nature of the ischemic event may be important in determining the degree of associated pain, we suggest more study of the individual who perceives the event. Myocardial ischemia may not generate a spontaneous report of chest pain because the patient is generally hyposensitive to visceral sensation; because he or she is coping with the threat of heart disease by denying the evidence of it--ie, denying the pain to deny the disease; or because the patient misunderstands the cause and significance of a vague or ambiguous cardiac sensation, normalizing the symptom and misattributing it to a nonpathologic cause.

Characterization and purification of a protease in serum that cleaves proatrial natriuretic factor (ProANF) to its circulating forms.

Atrial natriuretic factor (ANF) is synthesized and stored in atrial cardiocytes as a 17-kilodalton (kDa), 126 amino acid polypeptide, proANF, but circulates as smaller, 24 and 28 amino acid peptide fragments of the carboxy terminus of proANF. It has previously been shown that proANF is secreted intact from cultured atrial cardiocytes and can be cleaved by a serum protease to smaller, 3-kDa peptides believed to be the circulating forms. This report describes the purification and characterization of this proANF-cleaving protease from rat serum. The cleavages both of 35S-labeled proANF derived from rat atrial cell cultures, as assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE)/autoradiography, and of a synthetic p-nitroanilide-containing substrate were used as assays for the detection of enzyme activity. ProANF-cleaving activity was found in rat serum, with no such activity detectable in rat plasma. Cleavage in serum was not dependent on the presence of platelets or other cellular elements. Complete inhibition of proANF cleavage was obtained with the protease inhibitors benzamidine, leupeptin, phenylmethanesulfonyl fluoride, and diisopropyl fluorophosphate (DFP) but not with aprotinin, soybean trypsin inhibitor, pepstatin, or hirudin. Unlike the vitamin K dependent plasma proteins, the proANF-cleaving protease did not adsorb to barium sulfate. With the sequential application of ion-exchange, hydroxylapatite, lectin affinity, and gel filtration chromatography, a 5000-6000-fold purification of the enzyme from rat serum was achieved. Fractionation of either whole serum or the purified enzyme by gel filtration chromatography revealed a single peak of activity corresponding to a protein with a Stokes radius of 45 A.(ABSTRACT TRUNCATED AT 250 WORDS)

A serum protease cleaves proANF into a 14-kilodalton peptide and ANF.

Proatrial natriuretic factor (proANF), the 126-amino acid precursor of ANF, is the major storage form in mammalian atria. In contrast, two ANF peptides containing the 28- and 24-carboxyterminal residues of proANF have been isolated from rat plasma. Whether the cleavage of proANF in vivo to these ANF peptides occurs during or after its release into the circulation has not been determined. The latter possibility was suggested by our previous study where, by using a cultured rat cardiocyte preparation, we demonstrated that proANF is secreted intact into the culture medium. We now report that serum, but not plasma, contains a protease that specifically cleaves the 17-kdalton proANF to a 14-kdalton amino-terminal peptide and the carboxyterminal 3-kdalton circulating forms of ANF. The role of this proANF-cleaving enzyme in the generation of the biologically active ANF peptides remains to be defined. Its isolation and characterization should provide insights into its site of production and whether in vivo it is involved in the processing of circulating proANF.

Atrial natriuretic factor: assessment of its structure in atria and regulation of its biosynthesis with volume depletion.

Atrial natriuretic factor (ANF) is a polypeptide that has been isolated from mammalian atrial tissue with potent natriuretic, diuretic and spasmolytic properties. Details of its biosynthesis, cleavage, storage and release are not yet fully defined. Using sequence-specific anti-peptide antisera in immunocytochemical and radioimmunoassay studies, we demonstrated the presence in atrial granules of sequences corresponding to both the large molecular weight ("prohormone") form of ANF and the carboxyterminal peptide believed to be responsible for its biological activity. However, a peptide sequence spanning the proposed site of cleavage of the "signal peptide" from preproANF was undetected. In volume-depleted rats, a 28% decrease in the relative concentrations of atrial ANF mRNA was observed as compared to that of controls (P less than 0.01). There was, however, no significant difference observed between the two groups of animals with respect to atrial levels of ANF, as measured by radioimmunoassay. We conclude from these studies that proANF, but not preproANF, is stored in atrial granules. The concentration of stored ANF remains constant in volume-depleted rats despite a 28% reduction in ambient levels of ANF mRNA. The physiologic significance of these findings is discussed.