Ornithine decarboxylase in Pneumocystis carinii and implications for therapy

Pneumocystis carinii pneumonia (PCP) can be treated with eflornithine (difluoromethylornithine, DFMO, Ornidyl), a competitive irreversible inhibitor of ornithine decarboxylase (ODC), a key enzyme for polyamine biosynthesis. Because ODC has been reported to be absent from P. carinii, it has been assumed that eflornithine affects P. carinii only indirectly, by affecting host polyamine biosynthesis. If this is true, then improvements in the selectivity of antipolyamine therapy for PCP would be limited. Since the presence of ODC in P. carinii is an important issue, a new search for this enzyme was made. Not only were initial assays negative, but P. carinii extract reduced the background catalytic action of pyridoxal-5'-phosphate, the coenzyme required by the enzyme. This suggested the presence of an inhibitor, which was further supported by the observation that a P. carinii extract could suppress a source of known ODC activity. The inhibitory activity could be removed by a desalting column or by dialysis, allowing detection of P. carinii ODC. Indirect evidence indicates that the inhibition is only apparent and is caused by unlabeled ornithine in the extract of P. carinii which interferes with the radiolabel-based assay system. P. carinii and host ODCs respond differently to changes in pH. P. carinii ODC is much less susceptible to inhibition by eflornithine than host ODC. The presence of ODC in P. carinii suggests that P. carinii ODC is the target of eflornithine and that P. carinii ODC may have sufficiently specific properties that inhibitors with improved selectivity against P. carinii ODC could be identified.

Polyamine-dependent expression of the matrix metalloproteinase matrilysin in a human colon cancer-derived cell line

Matrilysin, which is a member of the matrix metalloproteinase family and is implicated in colon cancer invasion, is expressed in human colon adenocarcinoma-derived SW1116 cells. We investigated the effect of alpha-difluoromethylornithine (DFMO) on matrilysin expression in this cell line because others have shown that DFMO can inhibit invasion and carcinogenesis in epithelial tissues, including the colon, in experimental models. DFMO reduced extracellular levels of matrilysin protein after 4 d of treatment. Intracellular levels of matrilysin protein were minimally affected by DFMO treatment. The decrease in extracellular matrilysin protein levels caused by DFMO was not a consequence of lowered steady-state levels of matrilysin mRNA. After 4 d of exposure, the amount of this transcript was higher in DFMO-treated cells than in untreated cultures, whereas the mRNA stabilities were similar. These data show that polyamine depletion by DFMO can suppress the expression of matrilysin, a gene product thought to be involved in tumor invasion. The decrease in extracellular matrilysin protein caused by DFMO treatment appears to be due to a posttranscriptional mechanism, although transcription of this gene also seems to be affected by polyamines in SW1116 cells.

Regulation of N-methyl-D-aspartate receptor-mediated calcium transport and norepinephrine release in rat hippocampus synaptosomes by polyamines

The role of polyamines (PA) synthesis in NMDA receptor-mediated 45Ca2+ fluxes and norepinephrine release was studied in rat hippocampal synaptosomes. NMDA (50 microM) caused a sharp (> 2-fold) transient increase in PA synthesis regulating enzyme, ornithine decarboxylase (ODC) activity with concomitant elevation in PA levels in the order putrescine > spermidine > spermine. ODC inhibitor, alpha-difluoromethylornithine (DFMO), and NMDA antagonist, 2-amino-5-phosphonovaleric acid (D-AP5), both blocked increases in ODC activity and PA levels. Activation of NMDA receptors induced a sharp (3 to 4-fold) and quick (15 seconds) increase in 45Ca2+ uptake by synaptosomes within 15 seconds of exposure at 37 degrees C. The efflux of 45Ca2+ and 3H-norepinephrine (NE) release at 22 degrees C from pre-loaded synaptosomes was also significantly (2 to 4-fold) enhanced by NMDA within 15 seconds. These NMDA receptor-mediated effects on calcium fluxes and NE release were blocked by NMDA receptor-antagonists (DAP-5 and MK-801) and PA synthesis inhibitor, DFMO and the DFMO inhibition nullified by exogenous putrescine. These observations establish that ODC/PA cascade play an important role in transduction of excitatory amino acid mediated signals at NMDA receptors.

Regulation of putrescine uptake in Leishmania mexicana promastigotes

Putrescine uptake of Leishmania mexicana promastigotes is tightly regulated by polyamine intracellular concentrations. This uptake, markedly stimulated after parasite treatment with alpha-difluoromethylornithine (DFMO) for 48 to 72 hrs., was strongly repressed by exposure of Leishmania cultures to exogenous putrescine or its derivative 1,4-dimethylputrescine. In contrast, spermidine, spermine, diaminopropane and cadaverine were unable to decrease putrescine transport. Both, the uptake induction as well as its specific feedback repression by increased levels of endogenous putrescine requires protein synthesis since they were abolished after addition of cycloheximide for several hours. Our results seem to indicate that putrescine transporter is a stable and specific protein which can be reversibly inactivated by a relatively unstable repressor.

Blockade of ornithine decarboxylase enzyme protects against ischemic brain damage

Polyamines are derived from ornithine by the actions of ornithine decarboxylase (ODC), which is the rate-limiting step in this pathway. Polyamines play a role in cell growth, neoplasia, differentiation, and response to injury. We have shown that transient cerebral ischemia gives rise to increased ODC mRNA and enzyme activity in the gerbil brain. ODC and polyamines are thought to be important in the generation of edema and the neuronal cell loss associated with cerebral ischemia. To test this theory, we examined the ODC activity, putrescine levels, and neuronal density in the CA1 region of the hippocampus following ischemia and reperfusion injury in the absence and presence of an inhibitor of ODC activity, alpha-difluoromethylornithine (DFMO). Pretreatment of animals with DFMO resulted in attenuation of the ODC activity following 5 min of ischemia and 4 h of reperfusion. In addition, DFMO prevented the increase in polyamine levels, as determined by measurement of putrescine in the ischemic brain. These alterations were not due to changes in ODC mRNA level. Further analysis revealed that DFMO treatment blocked the delayed neuronal cell death in the CA1 region of the hippocampus that accompanies ischemia and reperfusion injury. Administration of DFMO resulted in a dose-dependent beneficial effect upon neuronal cell survival. These results suggest that ODC enzyme activity and the production of polyamines play a significant role in the response of the brain to ischemic injury.

Neurospora mutants affecting polyamine-dependent processes and basic amino acid transport mutants resistant to the polyamine inhibitor, alpha-difluoromethylornithine

Polyamines (spermidine and spermine) are required by living cells, but their functions are poorly understood. Mutants of Neurospora crassa with enhanced or diminished sensitivity to interference with polyamine synthesis, originally selected to study the regulation of the pathway, were found to have unexpected defects. A group of four non-allelic mutations, causing no interference with polyamine synthesis, each imparted spermidine auxotrophy to a genotype already partially impaired in spermidine synthesis. Strains carrying only the new mutations displayed unconditional delay or weakness at the onset of growth, but grew well thereafter and had a normal or overly active polyamine pathway. These mutants may have defects in vital macromolecular activities that are especially dependent upon the polyamines-activities that have not been identified with certainty in studies to date. Another group of mutants, selected as resistant to the polyamine inhibitor difluoromethylornithine (DFMO), had normal activity and regulation of ornithine decarboxylase, the target of the drug. All but one of thirty mutants were allelic, and were specifically deficient in the basic amino acid permease. This mechanism of DFMO resistance is unprecedented among the many DFMO-resistant cell types of other organisms and demonstrates that DFMO can be used for efficient genetic studies of this transport locus in N. crassa.

The role of hypusine depletion in cytostasis induced by S-adenosyl-L-methionine decarboxylase inhibition: new evidence provided by 1-methylspermidine and 1,12-dimethylspermine

The abilities of the natural polyamines, spermidine and spermine, and of the synthetic analogues, 1-methylspermidine and 1,12-dimethylspermine, to reverse the effects of the S-adenosyl-L-methionine decarboxylase inhibitor 5'-([(Z)-4-aminobut-2-enyl]methylamino)-5'-deoxyadenosine (AbeAdo) on L1210-cell growth were studied. L1210 cells were exposed to AbeAdo for 12 days to induce cytostasis and then exposed to spermidine, spermine, 1-methylspermidine or 1,12-dimethylspermine in the continued presence of AbeAdo. AbeAdo-induced cytostasis was overcome by the natural polyamines, spermidine and spermine. The cytostasis was also reversed by 1-methylspermidine. 1,12-Dimethylspermine had no effect on the AbeAdo-induced cytostasis of chronically treated cells, although it was active in permitting growth of cells treated with the ornithine decarboxylase inhibitor, alpha-difluoromethylornithine. The initial 12-day exposure to AbeAdo elevated intracellular putrescine levels, depleted intracellular spermidine and spermine, and resulted in the accumulation of unmodified eukaryotic translation initiation factor 5A (eIF-5A). Exposure of these cells to exogenous spermidine, which is the natural substrate for deoxyhypusine synthase, resulted in a decrease in the unmodified eIF-5A content. 1-Methylspermidine, which was found to be a substrate of deoxyhypusine synthase in vitro, also decreased the levels of unmodified eIF-5A in the AbeAdo-treated cells. Although spermine is not a substrate of deoxyhypusine synthase, spermine was converted into spermidine in the L1210 cells, and spermine addition to AbeAdo-treated cells resulted in the appearance of both intracellular spermine and spermidine and in the decrease in unmodified eIF-5A. Exogenous 1,12-dimethylspermine, which was not metabolized to spermine or to 1-methylspermidine and was not a substrate of deoxyhypusine synthase in vitro, did not decrease levels of unmodified eIF-5A. The finding that AbeAdo-induced cytostasis was only reversed by polyamines and polyamine analogues that result in the formation of hypusine or an analogue in eIF-5A is consistent with the hypothesis [Byers, Wiest, Wechter and Pegg (1993) Biochem. J. 290, 115-121] that AbeAdo-induced cytostasis is due to the depletion of the hypusine-containing form of eIF-5A, which is secondary to the depletion of spermidine by inhibition of S-adenosyl-L-methionine decarboxylase.

alpha-Difluoromethylornithine delays behavioral recovery and induces decompensation after unilateral labyrinthectomy

Biochemical and pharmacologic studies suggest a role for the ornithine decarboxylase-polyamine system as a modulator of behavioral changes during vestibular compensation. alpha-Difluoromethylornithine specifically blocks the rate-limiting step of polyamine biosynthesis. To assess the effects of alpha-difluoromethylornithine on the acute phase of postural compensation, guinea pigs were divided into groups subjected to either unilateral labyrinthectomy only (n = 7), alpha-difluoromethylornithine (500 mg/kg/day) for 4 days before labyrinthectomy (n = 10), equivalent volumes of saline for 4 days before labyrinthectomy (n = 8), and sham operations (n = 5). Yaw head tilt and roll head tilt, trunk curvature, and air-righting reflex were measured at baseline and at regular intervals up to 4 weeks. alpha-Difluoromethylornithine significantly delayed recovery of normal air-righting but had no effect on yaw head tilt, roll head tilt, and trunk curvature. We also evaluated effects of alpha-difluoromethylornithine in compensated guinea pigs. Fully compensated animals from phase 1 were randomly assigned to receive alpha-difluoromethylornithine (500 mg/kg/day) or saline once daily for 4 days. Only 33% of alpha-difluoromethylornithine animals maintained air-righting, compared with 100% of saline-treated animals (p = 0.003). Maximum trunk curvature was greater in the alpha-difluoromethylornithine group (p = 0.02). Thus alpha-difluoromethylornithine not only delayed the time course for postural recovery after unilateral labyrinthectomy, it also transiently disrupted the maintenance of the compensated state.

Effects of the S-adenosylmethionine decarboxylase inhibitor, 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine, on cell growth and polyamine metabolism and transport in Chinese hamster ovary cell cultures

The regulation of polyamine transport and the roles of polyamine transport and synthesis in cell growth were investigated using cultured Chinese hamster ovary (CHO) cells and CHOMG cells which are mutants lacking polyamine-transport activity. Metabolically stable methylated polyamine analogues were used to measure polyamine accumulation, and the irreversible S-adenosyl-L-methionine decarboxylase inhibitor, 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine (AbeAdo), was used to inhibit synthesis. Exposure to AbeAdo lead to a dose-dependent decrease in growth for both cell lines, although CHOMG cells were more sensitive. Intracellular putrescine levels were greatly increased in AbeAdo-treated CHO cells and to a lesser extent in CHOMG cells, whereas intracellular spermidine and spermine levels were substantially reduced in both. Treatment with AbeAdo increased putrescine content in the culture medium to a much greater extent in CHOMG cultures indicating that a portion of the excess putrescine synthesized in response to AbeAdo treatment is excreted, but that CHO cells salvage this putrescine whereas it is lost to CHOMG cells which cannot take up polyamines. AbeAdo treatment increased polyamine transport into CHO cells despite high intracellular putrescine, suggesting that spermidine and/or spermine, and not putrescine, are the major factors regulating transport activity. The accumulation of either 1-methylspermidine or 1,12-dimethylspermine was significantly increased by AbeAdo treatment. Accumulation was increased even further when protein synthesis was blocked by cycloheximide, indicating that a short-lived protein is involved in the regulation of polyamine uptake. In the presence of cycloheximide and AbeAdo or alpha-difluoromethylornithine, methylated polyamine derivatives accumulated to very high levels leading to cell death. These results show that the polyamine-transport system plays an important role in retaining intracellular polyamines and that down-regulation of the transport system in response to increased intracellular polyamine content is necessary to prevent accumulation of toxic levels of polyamines.

Precocious cessation of intestinal macromolecular transport and digestive enzymes development by prostaglandin E2 in suckling rats

The effect of repeated oral administration of prostaglandin analogue (dmPGE2) on intestinal macromolecular transport and digestive enzymes development were investigated in the suckling rats. By the administration of dmPGE2 for 7 days, precocious induction of maltase activity, depression of amylase activity and enhancement of trypsin activity in the pancreas occurred. Absorption of bovine IgG was dose dependently depressed by dmPGE2 treatments. The intestinal cessation was also observed in the adrenalectomized pups, but was not influenced by difluoromethyl ornithine administration. These results suggest that oral administration of PGE2 induces precocious maturation of the small intestine and exocrine pancreas and that the intestinal cessation is not directly related to ornithine decarboxylase activity in the suckling rats.

Post-transcriptional inhibition of ornithine decarboxylase induction by zinc in a difluoromethylornithine resistant cell line

Addition of Zn2+ to cell medium inhibited the induction of ornithine decarboxylase (ODC) activity in ODC overproducing L1210-DFMOr cells. A significant effect was observed at a concentration as low as 0.01 mM, however, a more marked inhibition was caused by the addition of 0.1 mM Zn2+. The inhibition of the induction of ODC activity was accompanied by a proportional decrease in the content of immunoreactive ODC protein, whereas the level of ODC mRNA, determined by a solution hybridization RNase protection assay, was not affected significantly. Instead, some acceleration of ODC turnover was observed. The addition of 0.1 mM Co2+ or Mn2+, but not of other divalent metal ions, also inhibited ODC induction; differently from Zn2+ however, these metals affected cell viability and/or cell growth. Removal of endogenous Zn2+ by a chelator also provoked a strong decrease of ODC induction, which was reversed by Zn2+. However, addition of Zn2+ in excess of the chelator proved to be markedly inhibitory. These results indicate that both a restricted Zn2+ availability and an enhanced presence of the metal can inhibit the induction of ODC in L1210-DFMOr cells.

Characterization of polyamine transport in rat aortic smooth muscle cells

Increased arterial wall polyamine content has been linked to intimal hyperplasia (IH) formation. Intracellular polyamine content may be regulated by a polyamine transmembrane transport mechanism, but the existence of such a system has not been demonstrated in systemic arterial smooth muscle cells. This study characterizes polyamine transport as found in rat aortic smooth muscle cells. Smooth muscle cells were isolated and cultured from Sprague-Dawley rat aortas. Polyamine transport was determined by adding [14C]-polyamines to the medium, calculating transport kinetic parameters, Vmax and Km. Competition studies with unlabeled polyamines and uptake in the presence of paraquat, a polyamine transport inhibitor, were done to test the specificity of the uptake system. We identified polyamine transporters in aortic smooth muscle cells which were temperature, concentration, and time dependent. Kinetic studies revealed that spermidine and spermine had greater affinity for the transporter(s) than putrescine (Km = 0.3, 0.3, and 3.7 microM respectively; P = 0.0001) while maximum uptake velocity was similar for all polyamines (26.6-31.0 pmole/mg protein/min). Inhibition of de novo polyamine synthesis upregulated polyamine transport 2.8-3.8 times (P = 0.0001) while transporter affinity (as reflected by Km) remained unchanged. Competition studies and paraquat treatment indicated the presence of two polyamine transporters: one shared by all polyamines, the other specific for spermine and spermidine. These data indicate that transmembrane polyamine transport occurs in arterial smooth muscle cells. Upregulation of this system may represent one control mechanism for IH development.

Polyamines influence transglutaminase activity and cell migration in two cell lines

Transglutaminases (TGAs) catalyze the cross-linking of proteins through formation of gamma-glutaminyl-epsilon-lysine bonds and incorporation of small-molecular-weight amines, including polyamines, into the gamma-glutamine sites of proteins. Tissue TGA has been shown to establish covalent cross-links between cytoskeletal proteins using polyamines as substrates, and protein-polyamine conjugates have been identified in a variety of cells. We have shown previously that polyamines are required for cell migration in IEC-6 cells [S. A. McCormack, M. J. Viar, and L. R. Johnson. Am. J. Physiol. 264 (Gastrointest. Liver Physiol. 27): G367-G374, 1993]. In this study, we explored the relationship between cell migration, polyamines, and tissue TGA activity in two cell lines and found that while both IEC-6 and Caco-2 cells required normal levels of polyamines to migrate across a denuded surface, tissue TGA activity responded differently to polyamine deficiency brought about by treatment with alpha-difluoromethylornithine (DFMO). DFMO is a specific and irreversible inhibitor of ornithine decarboxylase, a rate-limiting enzyme of polyamine biosynthesis. In IEC-6 cells, tissue TGA activity decreased significantly with DFMO treatment concurrent with a rise in inactive TGA protein as measured by Western blot analysis. On the other hand, in Caco-2 cells, tissue TGA activity and protein increased significantly with DFMO treatment. In both cell lines, addition of polyamines to the DFMO treatment restored cell migration, tissue TGA activity, and protein to control levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Ornithine decarboxylase is a mediator of c-Myc-induced apoptosis

c-Myc plays a central role in the regulation of cell cycle progression, differentiation, and apoptosis. However, the proteins which mediate c-Myc function(s) remain to be determined. Enforced c-myc expression rapidly induces apoptosis in interleukin-3 (IL-3)-dependent 32D.3 murine myeloid cells following IL-3 withdrawal, and this is associated with the constitutive, growth factor-independent expression of ornithine decarboxylase (ODC), a rate-limiting enzyme of polyamine biosynthesis. Here we have examined the role of ODC in c-Myc-induced apoptosis. Enforced expression of ODC, like c-myc, is sufficient to induce accelerated death following IL-3 withdrawal. ODC induced cell death in a dose-dependent fashion, and alpha-difluoromethylornithine (DFMO), an irreversible inhibitor of ODC enzyme activity, effectively blocked ODC-induced cell death. ODC-induced cell death was due to the induction of apoptosis. We also demonstrate that ODC is a mediator of c-Myc-induced apoptosis. 32D.3-derived c-myc clones have augmented levels of ODC enzyme activity, and their rates of death were also a function of their ODC enzyme levels. Importantly, the rates of death of c-myc clones were inhibited by treatment with DFMO. These findings demonstrate that ODC is an important mediator of c-Myc-induced apoptosis and suggest that ODC mediates other c-Myc functions.

Effects of metal treatment on DNA repair in polyamine-depleted HeLa cells with special reference to nickel

Human cells depleted of the naturally occurring polyamines putrescine, spermidine, and spermine exhibit altered chromatin structure and marked deficiencies in DNA replicative and repair processes. Similar effects have been observed following treatment of normal mammalian cells with various heavy metal salts. In an attempt to better understand how metals interfere with normal DNA metabolic processes, a series of studies was carried out in which the toxicity and repair-inhibitory properties of various metals were evaluated in polyamine-depleted HeLa cells. Cytotoxicity of copper, zinc, magnesium, and cadmium was not altered in cells carrying lower polyamine pools. However, the sensitivity to nickel was markedly increased upon polyamine depletion, a condition that was readily reversed by polyamine supplementation. Nucleoid sedimentation analysis indicated that a greater amount of nickel-induced DNA damage occurred in polyamine-depleted cells than in normal cells, possibly serving as the basis for the increased sensitivity. Both polyamine depletion and nickel treatment result in decreased repair of DNA strand breaks and decreased cloning efficiency following X-ray and ultraviolet irradiation. Nickel treatment of polyamine-depleted cells resulted in synergistic sensitivity to both radiation treatments. None of the other metals tested enhanced X-ray or ultraviolet sensitivity of polyamine-depleted cells. Analysis of retarded repair sites following ultraviolet irradiation indicated those sites to be nonligatable in polyamine-depleted and nickel-treated cells, suggesting a block in the normal gap-sealing process.

Polyamine deficiency causes reorganization of F-actin and tropomyosin in IEC-6 cells

In earlier work we have shown that polyamine-deficient IEC-6 cells lose most of their ability to migrate. In this report we describe the effect of polyamine deficiency on the cytoskeleton of migrating IEC-6 cells. Cells were grown on cover slips for 4 days. One-third of the monolayer was removed, and the remainder was incubated for 6 h. The monolayers were fixed and stained with rhodamine phalloidin for actin filaments and by immunocytochemistry for tropomyosin. In control cells, actin filaments were found as stress fibers traversing the cell, in a thin actin cortex often visible on only one edge of the cell, and in fine fibers extending into the lamellipodia. Tropomyosin was found in the same distribution. A Western blot showed that tropomyosin was present as 35- and 37-kDa isoforms. In polyamine-deficient cells, actin stress fibers were less dense, whereas the actin cortex was greatly increased in density and lamellipodia were less extensive. Tropomyosin distribution was similar and included a 30-kDa isoform not seen previously. In spite of the obvious changes in the distribution of these cytoskeletal proteins, the concentrations of filamentous actin, beta-actin mRNA, and the higher molecular weight tropomyosin isoforms did not change. In all cases the addition of putrescine to polyamine-deficient cells prevented the changes described. We conclude that polyamines are essential for migration in this system because of their effects on the organization of cytoskeletal actin, tropomyosin, and perhaps other proteins as well.

Antizyme delays the restoration by spermine of growth of polyamine-deficient cells through its negative regulation of polyamine transport

Effects of antizyme on polyamine transport and spermine restoration of the growth of polyamine-deficient cells were examined by using mouse FM3A cells transfected with pMAMneoZ1 possessing rat antizyme cDNA under the control of glucocorticoid-inducible promoter. Treatments of the transfected cells with alpha-difluoromethylornithine (DFMO), an inhibitor of ornithine decarboxylase (ODC), and dexamethasone, an inducer of antizyme, both caused a decrease in ODC activity and polyamine contents and inhibition of cell growth. However, spermine uptake of the transfected cells was repressed by dexamethasone but stimulated by DFMO. The decrease in the rate of spermine uptake in dexamethasone-treated cells was attributed to an increase in Km value and a slight decrease in Vmax value. Accordingly, restoration of cell growth by spermine was less effective in dexamethasone-treated cells than DFMO-treated cells. These results clearly indicate that antizyme has dual functions: one for ODC degradation and the other for negative regulation of polyamine transport.

Early life-stage effects in medaka (Oryzias latipes) following in ovo exposure to polyamine biosynthetic inhibitors

Medaka, Oryzias latipes, were exposed in ovo to the polyamine (PA) biosynthesis inhibitors alpha-difluoromethylornithine (DFMO) and methylglyoxal bis(guanylhydrazone) (MGBG). In an additional group, spermine, the end product of the PA pathway, was added with DFMO and MGBG for a "rescue" treatment. At 4 days posthatch, length, DNA and RNA content, and swimming endurance were measured. The only parameter affected by treatment was swimming endurance which revealed decreased latent time to fatigue with increased dose, although not statistically significant. The rescue group, however, did demonstrate a statistically significant decrease in fatigue latency as compared to controls.

Polyamine effects on [Ca2+]i homeostasis and contractility in isolated rat ventricular cardiomyocytes

In electrically stimulated myocytes loaded with the fluorescent Ca2+ indicator indo 1-acetoxymethyl ester, spermine induced a dose-dependent (100-500 microM) negative inotropic effect, which was associated with a decrease in the magnitude of the cytosolic Ca2+ concentration ([Ca2+]i) transient but not with changes in myofilament responsiveness to Ca2+. Spermidine induced a less pronounced negative inotropic effect, whereas putrescine did not modify myocyte contraction. In the unstimulated state, spermine did not alter resting [Ca2+]i. Superfusion of the cardiac myocytes with 10 mM alpha-difluoromethylornithine, an inhibitor of polyamine synthesis, did not modify cellular responses to isoproterenol (10(-9)-10(-7) M). beta-Adrenergic stimulation did not affect either ornithine decarboxylase activity or intracellular polyamine levels within a 10-s to 15-min period of treatment. In summary, only exogenously administered polyamines were able to influence myocyte contractility. Their negative inotropic effect resulted from changes in [Ca2+]i homeostasis and required cellular depolarization.