Sympathetic innervation modulates ventricular impulse propagation and repolarisation in the immature rat heart

OBJECTIVE

When cardiac sympathetic innervation in neonatal rats is retarded by antiserum to nerve growth factor, there is a corresponding increase in the QT interval on ECG. Since the propagation of the cardiac impulse and the repolarisation of cardiac cells both contribute to the QT interval, the aim of this study was to determine the role of sympathetic innervation in modulating ventricular impulse propagation and repolarisation.

METHODS

Neonatal rats were treated for the first 10 days of life with nerve growth factor (NGF), its antiserum (As), or placebo. Standard microelectrode techniques were used to study the transmembrane action potential characteristics of subendocardial (ventricular septal) and subepicardial ventricular myocardium. Bipolar surface electrograms were used to record the velocity of impulse propagation and electron microscopy to examine the intercalated discs.

RESULTS

In the subendocardium, the phase 0 upstroke velocity of the action potential (dV/dtmax) was lowest in the As treated rats. The latter group also showed the slowest conduction velocity. There were no differences in control action potential durations in the endocardium among the three groups, but in the epicardial tissues, action potential duration was longest in the As treated group. Thus the dispersion in action potential duration was smallest in the As treated animals. Electron microscopic studies of the intercalated discs of ventricular myocytes showed significant enhancement of nexal junction formation in NGF treated rats, whereas As treated animals showed a retarded pattern of both nexal and desmosomal junction formation.

CONCLUSIONS

The differences in ultrastructure, conduction, and repolarisation seen in As and NGF treated animals may explain the prolonged QT interval seen in the As treated group.

[Transcervical resection of endometrium and submucous myomas]

Transcervical resection of the endometrium and submucous myomas using an unmodified urologic resectoscope has recently been developed as a treatment for menorrhagia and/or infertility when conservative management proves unsatisfactory but the patient desires to preserve the fertility or wishes to avoid hysterectomy. Through June 1990, 7 patients were treated with this procedure including one for endometrial ablation and six for transcervical myomectomy alone. No significant complication was seen and the results were relatively satisfactory in an average 7 1/2 months follow up. This approach seems a safe, effective surgical procedure and represents an attractive alternative to hysterectomy or abdominal myomectomy.

Scrub typhus and pregnancy: a case report and literature review

Scrub typhus is still prevalent on Penghu Islands. We herein report a case in a pregnant woman who had been to Yi-Lan County in Taiwan. The patient responded well to Minocycline (Minocin) therapy without complication. Her newborn baby was found not complicated with the disease. The relative literature is reviewed.

Chronic exposure to neuropeptide Y determines cardiac alpha 1-adrenergic responsiveness

The onset of sympathetic innervation induces a developmental change in the cardiac alpha 1-adrenergic chronotropic response from an increase to a decrease in rate. The mechanism by which innervation induces this alteration is unknown. Neuropeptide Y (NPY), which is found abundantly in cardiac sympathetic nerve terminals, was considered as a possible mediator for this effect. Chronic conditioning by NPY in noninnervated myocyte cultures stimulated the effect of sympathetic innervation in inducing the alpha 1-inhibitory chronotropic response. Chronic conditioning by the NPY antagonist PYX-2 blocked the effect of innervation. Thus endogenous NPY may modulate alpha 1-adrenergic responsiveness during the ontogeny of cardiac sympathetic innervation.

The electrophysiologic effects of bupivacaine on adult, neonatal, and fetal guinea pig papillary muscles

The authors used standard microelectrode techniques to study developmental changes in the effects of bupivacaine on the transmembrane potentials of adult, neonatal, and fetal guinea pig papillary muscles. Bupivacaine hyperpolarized membrane potential in the adult and neonatal muscles but not the fetal muscles. In all three age groups, action potential overshoot and the maximum rate of increase of phase 0 (Vmax) were significantly reduced by bupivacaine greater than or equal to 1.0 microgram/ml. Bupivacaine 1.5 micrograms/ml reduced action potential duration at both 50% and 100% repolarization in the fetal tissues, but not in adult or neonatal tissues. Tonic block was not induced by bupivacaine in any of the three groups. Use-dependent block was variable at bupivacaine 0.2 micrograms/ml in all three groups and was consistent and equivalent at higher concentrations. The onset and offset of use-dependent block were the same in all three groups, with onset occurring between 6.0 and 6.7 beats and the time constant for recovery being 1.9-2.3 s. The authors conclude there is an age-related bupivacaine effect on action potential duration but no age-related change in bupivacaine-induced use-dependent block.

[Observation on phrenic discharges induced by bicuculline or picrotoxin in spinal rabbits]

The respiratory activity of spinal cord-transected animals was reinvestigated on 71 rabbits. Under light urethane anesthesia and paralyses by gallamine, the spinal cord was transected at either C1 or C2 level, and phrenic discharges were monitored in 43 rabbits. After cordotomy, tonic phrenic discharges were observed in 25 of the 43 spinal rabbits. No spontaneous rhythmic phrenic activities appeared in any of the animal. Respiratory-like, long lasting phrenic bursts (lasting longer than 0.25s) or convulsive-like, short lasting phrenic bursts (lasting shorter than 0.1s on the average) were induced by administration of bicuculline (BCL, 20 micrograms/10 microliters-40 micrograms/20 microliters, intra-subarachnoid space, i.s.s.) in 24 spinal rabbits or picrotoxin (PIC, 20 micrograms/20 microliters, i.s.s. or 3-5 mg/kg i.v.) in 13 spinal rabbits. According to the pattern of the recruitment and de-recruitment of the phrenic discharges, the long lasting phrenic bursts may be divided into three types: Type I, average frequency 23.5 +/- 2.3 cycles/min (BCL), incidence 58.5% (BCL) or 67.7% (PIC); Type II, average frequency 33.8 +/- 4.7 cycles/min (BCL), incidence 39.3% (BCL) or 32.3% (PIC); Type III, average frequency 21.3 +/- 2.8 cycles/min (BCL), incidence 2.2% (BCL) or 3.3% (PIC). The duration of the evoked phrenic discharges was 60.0 +/- 18.9 min (BCL) or 42.0 +/- 0.8 min (PIC). The type I and type II showed respiratory-like discharges, accounting for more than 97.8% of the incidence of the long lasting phrenic bursts. It is suggested that the endogenous GABA system in spinal cord might exert a tonic inhibitory action on the spinal "respiration" activity.

Sympathetic neural modulation of cardiac impulse initiation and repolarization in the newborn rat

We injected neonatal rats with nerve growth factor, the antiserum to nerve growth factor, or placebo for the first 10 days of life. Our goal was to determine the relation between sympathetic innervation of the developing heart, the electrocardiographic expression of cardiac rhythm, and the response of the heart to alpha-adrenergic stimulation with phenylephrine. We were especially interested in the latter area because of the prior demonstration in isolated cell systems of sympathetic neural modulation of a 41-kDa GTP regulatory protein and alpha-adrenergic responsiveness. Ten- to 11-day-old rats treated with nerve growth factor had more complete sympathetic innervation, faster heart rates, and higher levels of the 41-kDa protein than the placebo group. Electrophysiological studies were performed on isolated ventricular septa superfused with Tyrode's solution at 37.0 degrees-37.5 degrees C. The electrophysiological response of septa to 10(-9) and 10(-8) M phenylephrine from the 10-11-day-old nerve growth factor group was comparable with that of 3-week-old control animals. In contrast, 10-11-day-old antiserum-treated rats had an abnormal innervation pattern, lower levels of the 41-kDa protein, and a more immature electrophysiological response to alpha-adrenergic stimulation than the placebo group. In addition, antiserum-treated rats had an abnormally prolonged electrocardiographic QT interval. Our results demonstrate for the first time in intact animals a direct link between sympathetic innervation and alpha-adrenergic receptor-effector coupling as well as the dependence on innervation of the modulation of impulse initiation by alpha-agonists. This sequence of developmental events may be important not only in the regulation of normal cardiac rhythm but also in the expression of certain pathological entities such as the congenital long QT syndrome and the sudden infant death syndrome.

The positive chronotropic effect of acetylcholine has muscarinic and nicotinic components in the neonatal rat heart

Acetylcholine increases ventricular automaticity in neonatal but not adult canine Purkinje fibers. In this study, we used a rat model to investigate the mechanism for the increased automaticity, and used surface electrodes to record spontaneous rates from the ventricular septa of three different age groups: 1 to 2 days old (neonates), 6 to 9 days old (1 week old) and adults. Acetylcholine, 10(-12) and 10(-11) M, induced a significant increase in automaticity from a control of 103 +/- 6.5 beats per min to 117 +/- 9.0 and 118 +/- 10.8 beats per min, respectively, in the neonates (P less than .05). The increase was attenuated by atropine, 2 x 10(-6) M (P = .05), and eliminated by propranolol, 2 x 10(-7) M, or hexamethonium, 5 x 10(-6) M (P less than .05). In 1-week-old rats, acetylcholine, 10(-12) M, induced a lesser increase in automaticity from a control of 106 +/- 13.0 to 113 +/- 14.0 beats per min (P less than .05). The increase was blocked by atropine, 2 x 10(-6) M, propranolol, 2 x 10(-7) M, and by hexamethonium, 5 x 10(-6) M (all P less than .05). In adults, acetylcholine did not increase automaticity. Among the neonatal septa, 82% showed increased automaticity with acetylcholine alone, 78% showed increased automaticity in the presence of atropine and 13% showed increased automaticity in the presence of either propranolol or hexamethonium, suggesting a largely nicotinic mediated and catecholamine dependent component. In 1-week-old septa, 75% showed increased automaticity with acetylcholine alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of techniques for the selection of bacteria-free sperm preparations

The authors compared the three most commonly used sperm preparation techniques--swim-up, fall-down, and Percoll gradient--for their ability to recover highly motile sperm and minimize bacterial contamination. Eleven human semen samples collected by masturbation were used and run in parallel with the three methods. A semiquantitative bacterial analysis was performed in all samples and results expressed in colony-forming units per milliliter (CFU/ml). The Percoll gradient technique resulted in an average sperm concentration of 5.81 +/- 4.4 X 10(6) ml, and the average bacterial concentration dropped from 8.66 +/- 12.96 X 10(3) CFU/ml in semen to 0.01 +/- 0.03 X 10(3) CFU/ml. The bacterial count was not significantly different when the raw semen was compared with the swim-up or the fall-down preparations. The authors conclude that the Percoll gradient method yields an adequate sperm concentration, with high motility and improved morphology, while eliminating bacterial contamination.