Synapse formation and spontaneous activity in rat brainstem neurons in primary culture

The correlation between synaptogenesis and onset of spontaneous action potentials was assessed in rat brainstem cells up to 29 days in primary culture. Cells exhibited different stages of maturation followed by electron microscopy and patch clamp recordings. Terminal boutons with no preferential orientation of presynaptic vesicles appeared after 2 days in culture. After 5 days, preferential orientation of presynaptic vesicles and thickening of postsynaptic membranes were observed. The spontaneous discharge of action potentials, single or bursting, was observed after 7 days in vitro. This was followed by the expression of a 128-pS K(+) channel starting at 13 days in vitro. A 69-pS K(+) channel was also present throughout the duration of the cultures. These results suggest that spontaneous discharge of action potentials does not occur before synapses are formed and K(+) channel types develop differentially in brainstem neurons in vitro.

Doxorubicin induces slow ceramide accumulation and late apoptosis in cultured adult rat ventricular myocytes

OBJECTIVES

Anthracyclines cause apoptotic death in many cell types through activation of the ceramide pathway. We tested the hypothesis that doxorubicin induces cardiac myocyte apoptosis through ceramide generation.

METHODS

Adult rat ventricular myocytes were grown in the presence of 10% fetal calf serum, and exposed to 0.5 microM doxorubicin (Dox) for 1 h on the day of cell isolation (day 0). We used the membrane-permeant ceramide analog C2-ceramide (C2-cer) to mimic the effects of endogenous ceramide and PDMP to induce endogenous ceramide accumulation. Apoptosis was assessed upon morphological criteria and DNA fragmentation by the TUNEL method and agarose gel electrophoresis. Ceramide concentration was assessed using the DAG kinase assay.

RESULTS

Myocyte exposure to Dox was associated with cellular and nuclear alterations typical of apoptosis on day 7 but not on day 3. At day 7, the percentage of TUNEL-positive myocytes was markedly increased in Dox-treated cultures compared to control (Cl) cultures (82 +/- 3 vs. 12 +/- 1%, n = 7; p < 0.001); internucleosomal DNA fragmentation was confirmed by the observation of DNA ladders. These alterations were associated with an increase in the intracellular ceramide concentration (1715 +/- 243 vs. 785 +/- 99 pmol/mg prot, n = 5; p < 0.01), a phenomenon also detected on day 3 (731 +/- 59 vs. 259 +/- 37 pmol/mg prot, n = 5; p < 0.001). Incubation of myocytes at day 0 with 50 microM C2-cer induced rapid cell shrinkage and DNA fragmentation (45 +/- 3 vs. 10 +/- 1% TUNEL-positive myocytes on day 1 in C2-cer-treated and Cl cultures, respectively; n = 6, p < 0.001). Myocyte exposure to 10 microM PDMP for 7 days (n = 5), caused ceramide accumulation (1.7-fold increase vs. Cl, p < 0.01), and a marked increase in the percentage of TUNEL-positive myocytes (62 +/- 6 vs. 11 +/- 3% in Cl cultures, p < 0.001). Ventricles of rats injected intraperitoneally with a cumulative dose of 14 mg/kg Dox over a period of 2 weeks also showed an increased ceramide concentration 2 weeks later (11.01 +/- 0.64 vs. 5.24 +/- 0.88 pmol/mg prot, n = 6; p < 0.001).

CONCLUSION

Our study confirms the existence of a functional ceramide pathway related to apoptosis in cardiac myocytes, and points to its possible involvement in doxorubicin-induced cardiomyopathy.

Influence of the endothelium, nitric oxide and serotonergic receptors on coronary vasomotor responses evoked by ergonovine in conscious dogs

1. The respective contributions of coronary vascular endothelium, nitric oxide (NO) and serotonergic receptors to the effects of ergonovine on large and small coronary arteries were investigated in conscious dogs. 2. In seven dogs with an endothelium intact, ergonovine (30 - 1000 microg, i.v.) induced a biphasic response on large coronary artery with an early and transient vasodilatation (up to +2.9+/-0.5% from 3310+/-160 microm, P<0.01) followed by a sustained vasoconstriction (down to -4.9+/-0.5%, P<0.001) which occurred simultaneously with a sustained increase in coronary blood flow (CBF) (up to +100+/-26% from 28+/-4 ml min(-1), P<0.001). After endothelium removal (balloon angioplasty), the ergonovine-induced vasodilatation was abolished and vasoconstriction potentiated (-6.4+/-0.9% after vs -4.9+/-0.5% before endothelium removal, P<0.01). 3. After blockade of NO synthesis by Nomega-nitro-L-arginine (30 mg kg(-1)) in four other dogs, the early vasodilatation induced by ergonovine was abolished but the delayed vasoconstriction as well as the increase in CBF remained unchanged. 4. Both ketanserin and methiothepin (0.3 mg kg(-1)) abolished the early vasodilatation and reduced the delayed vasoconstriction induced by ergonovine. Ketanserin decreased and methiothepin abolished the reduction in coronary resistance induced by ergonovine. 5. Thus, the complex interactions between vascular endothelium and serotonergic receptors to ergonovine-induced constriction of large coronary arteries might explain the induction of coronary spasms in patients with endothelial dysfunction.

Early redistribution of plasma membrane phosphatidylserine during apoptosis of adult rat ventricular myocytes in vitro

In many cell types, DNA fragmentation is a late event of apoptosis which may be lacking. This contrasts with the early translocation of phosphatidylserine (PS) from the internal to the external leaflet of the cell membrane. We examined whether an early PS translocation also occurs during apoptosis induced in adult rat ventricular myocytes grown in the presence of 10% fetal calf serum (FCS), by the protein kinase inhibitor staurosporine. Apoptosis was assessed by the observation of: (i) typical alterations in cell morphology; (ii) nuclear alterations visualized using the permeant intercalating agent Hoechst 33258; (iii) DNA fragmentation detected by the TUNEL method. PS translocation was detected using annexin V binding. Data are expressed as means +/- SEM. Prolonged exposure of myocytes to 10 microM staurosporine from day 3 to day 7 of culture resulted in cell shrinkage, typical nuclear alterations, membrane protrusions and fragmentation of the sarcomeric apparatus in the vast majority of myocytes. At this time, 52.4 +/- 5.7% of staurosporine-treated myocytes were TUNEL positive (vs 6.1 +/- 2.0% in control cultures (CC), p < 0.001) and 69.7 +/- 1.7% were annexin V positive (vs 21.1 +/- 1.0% in CC, p < 0.001). Importantly, PS translocation was detected as early as 35 minutes following staurosporine addition, the percentage of annexin V positive myocytes reaching 10 times the control value (19.2 +/- 2.7 vs 1.8 +/- 0.8%, p < 0.001) after 3 hours. A 18-hour staurosporine exposure of freshly isolated myocytes resulted, at the end of exposure, in 24.3 +/- 1.7% annexin V positive myocytes (vs 9.6 +/- 0.5% in CC, p < 0.05), whereas a marked increase in the percentage of TUNEL positive myocytes was observed only from day 5. Finally, myocyte exposure to the membrane-permeant ceramide analog, C2-ceramide (50 microM), resulted in 63.2 +/- 3.5% annexin V positive myocytes 4 hours later (vs 17.8 +/- 4.4% in CC, p < 0.001), whereas a significant increase in the percentage of TUNEL positive myocytes was detected only the next day (43.7 +/- 3.4 vs 9.9 +/- 1.3%, p < 0.001). Taken together, these results strongly suggest that the loss of PS asymmetry is an early event of cardiac myocyte apoptosis which precedes DNA fragmentation.

Effects of cyclic GMP elevation on isoprenaline-induced increase in cyclic AMP and relaxation in rat aortic smooth muscle: role of phosphodiesterase 3

1. In rat aortic rings precontracted with phenylephrine, the beta-adrenoceptor agonist isoprenaline (10 nM to 30 microM) produces greater relaxant effects in preparations with endothelium than in endothelium-denuded preparations. The aim of this study was to determine the mechanisms involved in this effect and in particular investigate the possibility of a synergistic action between adenosine 3':5'-cyclic monophosphate (cyclic AMP) and guanosine 3':5'-cyclic monophosphate (cyclic GMP). 2. Isoprenaline-induced relaxation of rat aortic rings precontracted with phenylephrine was greatly reduced by the nitric oxide (NO) synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME, 300 microM) or the soluble guanylate cyclase inhibitors methylene blue (10 microM) or IH-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 10 microM) but unaffected by indomethacin (10 microM), a cyclo-oxygenase inhibitor. Similarly, in intact rings, the concentration-response curve of forskolin (10 nM to 1 microM) was shifted to the right upon endothelium removal or treatment with methylene blue. 3. In endothelium-denuded rat aortic rings, isoprenaline-induced relaxation was potentiated by the guanylate cyclase activators atrial natriuretic factor (ANF, 1 to 10 nM) and sodium nitroprusside (SNP, 1 to 10 nM), and to a greater extent in the presence of the cyclic GMP-specific phosphodiesterase (PDE 5) inhibitor, 1,3dimethyl-6-(2-propoxy-5-methane sulphonylamidophenyl) pyrazolo [3,4-d] pyrimidin-4-(5H)-one (DMPPO, 30 nM). Relaxation induced by isoprenaline was also potentiated by the cyclic GMP-inhibited PDE (PDE 3) inhibitor cilostamide (100 nM). 4. Intracellular cyclic nucleotide levels were measured either in rat cultured aortic smooth muscle cells or in de-endothelialized aortic rings. In both types of preparation, isoprenaline (5 nM and 10 microM) increased cyclic AMP levels and this effect was potentiated by cilostamide (10 microM), by rolipram, a cyclic AMP-specific PDE (PDE 4) inhibitor (10 microM) and by cyclic GMP-elevating agents (50 nM ANF or 30 nM SNP plus 100 nM DMPPO). In isoprenaline-stimulated conditions, the increase in cyclic AMP induced by rolipram was further potentiated by cilostamide and by cyclic GMP-elevating agents. Cilostamide and cyclic GMP-elevating agents did not potentiate each other, suggesting a similar mechanism of action. 5. We conclude that in vascular smooth muscle (VSM) cells an increase in cyclic GMP levels may inhibit PDE 3 and, thereby, cyclic AMP catabolism. Under physiological conditions of constitutive NO release, and to a greater extent in the presence of the PDE 5 inhibitor DMPPO, cyclic GMP should act synergistically with adenylate cyclase activators to relax VSM.

Cardiovascular effects of a novel, potent and selective phosphodiesterase 5 inhibitor, DMPPO: in vitro and in vivo characterization

1. The aim of this study was to investigate the cardiovascular effects of a novel, potent and specific phosphodiesterase 5 (PDE 5) inhibitor, 1,3 dimethyl-6-(2-propoxy-5-methane sulphonylamidophenyl)-pyrazolo[3,4-d]pyrimidin-4-(5H)-one (DMPPO) in phenylephrine-precontracted rat aortic rings and different in vivo rat preparations. 2. DMPPO elicited a concentration-dependent relaxation of rat aortic rings with functional endothelium. DMPPO-induced relaxation was abolished by endothelium removal or pretreatment with the soluble guanylate cyclase inhibitor, methylene blue (10 microM). 3. In aortic rings without endothelium, the potency (pD2= -log10 EC50) of atrial natriuretic peptide (ANP) to induce relaxation increased from 8.13 +/- 0.05 in the absence of DMPPO, to 8.32 +/- 0.05 and 8.52 +/- 0.08 in the presence of 30 nM and 100 nM DMPPO, respectively. Similarly, the potency of sodium nitroprusside (SNP) in inducing relaxation increased from 7.38 +/- 0.07 in the absence of the PDE 5 inhibitor to 8.07 +/- 0.11 and 8.15 +/- 0.08 in the presence of 30 nM and 100 nM DMPPO, respectively. In contrast, relaxation to the adenylate cyclase activator, forskolin, was unchanged by DMPPO (100 nM). 4. In rings without endothelium, DMPPO (100 nM) increased by 2.5 fold intracellular levels of guanosine 3':5'-cyclic monophosphate (cyclic GMP). Moreover, DMPPO (100 nM) potentiated the increases in cyclic GMP levels induced by ANP (30 nM) by 3 fold and SNP (30 nM) by 2.7 fold. Adenosine 3':5'-cyclic monophosphate (cyclic AMP) levels were not modified by DMPPO. 5. In anaesthetized normotensive or spontaneously hypertensive rats (SHR), DMPPO (2 and 5 mg kg-1, i.v.) lowered blood pressure without affecting heart rate. Similarly, in conscious SHR, orally administered DMPPO (5 mg kg-1) induced a 25 mmHg decrease in blood pressure for at least 7 h without modifying heart rate. Meanwhile, urinary cyclic GMP was increased by 50% whereas cyclic AMP remained unchanged. 6. In normotensive anaesthetized rats, sodium nitroprusside (SNP) (i.v. bolus) induced a decrease in blood pressure which rapidly returned to baseline. In DMPPO (1 mg kg-1, i.v.)-treated rats, the hypotensive effects of SNP (10 to 100 micrograms kg-1) were prolonged over time whereas the peak effect was unchanged. 7. In pithed rats, phenylephrine (i.v. bolus) induced dose-dependent increases in blood pressure. Pretreatment with DMPPO (5 mg kg-1, i.v.) partially inhibited the pressor response to phenylephrine (0.3 to 100 micrograms kg-1). 8. In conclusion, the potent and selective PDE 5 inhibitor, DMPPO, produces relaxation in isolated vessels in the presence of a cyclic GMP drive and reduces blood pressure of intact animals. Its high oral bioavailability and long duration of action should make it a useful tool to study the role of cyclic GMP in various biological systems.