Methacholine-induced bronchoconstriction and phrenic motoneuron activity in dogs

To evaluate the effect of changes in bronchomotor tone on the firing pattern of single phrenic motoneurons, efferent activity of both the C5 branch phrenic nerve and a single fiber from the contralateral phrenic nerve were studied in dogs anesthetized (chloralose/urethane) and breathing spontaneously (65% O2). Bronchoconstriction was induced by inhalation of methacholine (MCh) aerosol, which increased total lung resistance, decreased dynamic lung compliance and tidal volume. The rate of rise of phrenic activity was significantly increased by the changes in pulmonary mechanics induced by MCh. Single phrenic motoneurons could be separated into early (EOM) and late (LOM) onset types based on the onset time of firing relative to the beginning of whole phrenic nerve activity. The increased phrenic nerve activity was due primarily to the recruitment of LOMs. The recruitment of LOMs consisted of a shift in the time of onset to an earlier portion in inspiration, a longer firing duration, and an increase in the number of spikes per inspiratory cycle. After bilateral cervical vagotomy, all the responses of phrenic nerve to changes in pulmonary mechanics induced by aerosolized MCh were attenuated. These results suggest that phrenic motoneurons activity is modulated by alterations in pulmonary mechanics via a reflex mediated by the vagus nerve and that LOM play an important part in this response.

Responses of single phrenic motoneurons to altered ventilatory drives in anesthetized dogs

We studied the effects of altered ventilatory drives on the activity of the whole phrenic nerve and single phrenic motoneurons in dogs anesthetized with alpha-chloralose and paralyzed with gallamine triethiodide. Single phrenic motoneurons were classified as either late-onset or early-onset motoneurons (LOM and EOM, respectively), depending on the time of onset of their activity during inspiration. Increase in ventilatory drive was induced by altering chemical drive with changes in arterial blood gases and also by altering the vagal afferent contribution to ventilatory drive. The latter was accomplished by inducing pulmonary gas embolism (PGE) during hyperoxia. Whole phrenic nerve activity was increased by both types of increase in ventilatory drive. In both cases, changes in the firing pattern of LOMs and EOMs were responsible for the increased phrenic output. The changes in post-PGE firing pattern of the LOMs generally consisted of a shift in the time of onset to an earlier point in inspiration and an increase in the number of spikes per inspiratory cycle. Vagotomy abolished the difference between the contributions of LOMs and EOMs to the phrenic response to PGE. Data from dogs studied while they were breathing spontaneously were qualitatively the same as those from the paralyzed animals, indicating no major role for phasic volume feedback in these responses. Our data regarding altered chemical drive are similar to those reported earlier in other species, whereas those regarding PGE demonstrate that vagally mediated increases in ventilatory drive affect both LOMs and EOMs, although LOMs are affected to a greater degree.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitation of herpes simplex virus in rabbit corneal epithelium

The authors have developed an objective method for quantitation of herpes simplex virus in the corneal epithelium of rabbits. At appropriate times postinfection, full-thickness rabbit corneas were removed by trephination and subjected to one cycle of freezing and thawing. The corneal epithelium was then disrupted by sonication. The amount of infectious virus recovered from sonicated specimens was determined by an in vitro plaque assay, providing a measure of the quantity of virus present during the acute stage of herpetic keratitis. Using this technique, the authors found that the mean virus titer was reduced from 1.5 X 10(5) plaque forming units (pfu) per cornea in control rabbits to less than 200 pfu per cornea in rabbits treated topically for 2 days with 1% trifluridine. In contrast, instillation of 1% prednisolone acetate resulted in the persistence of higher levels of virus (275 pfu) than those observed in control rabbits (3 pfu) 4 days after the cessation of therapy.

Effect of suprofen on corneal wound healing

We studied the effect of suprofen, a new ophthalmic nonsteroidal anti-inflammatory agent, on corneal wound healing. Nine-millimeter, central, perforating corneal wounds were made in albino rabbits and sutured with 10-0 nylon. The animals were randomly treated with balanced salt solution, suprofen vehicle, 1% suprofen, or 0.1% dexamethasone sodium phosphate administered topically for six days. On the seventh postoperative day, the sutures were removed and, in situ, the intraocular pressure was increased in a controlled manner until the wound burst. Dexamethasone applied four times a day significantly inhibited corneal wound healing, whereas suprofen given as often as hourly did not. Pretreatment with hourly administered suprofen for two days prior to surgery, in addition to the same postoperative hourly therapy, also did not significantly decrease stromal wound strength.

Breathing near to residual volume with positive expiratory pressure

Breathing near to residual volume produces arterial desaturation demonstrable by ear oximetry. This paper describes the effect on arterial saturation measured by an ear oximeter of expiring to residual volume, taking a normal inspired volume of air from ambient pressure, and then expiring to residual volume against a pressure of 10-15 cm of water. This breathing pattern was continued for several minutes. Carbon dioxide output and oxygen consumption were measured during the study and the closing volume at ambient pressure was compared with that at increased expiratory pressure. Arterial desaturation was actually increased by the increased airways pressure, the increase being modified both by minute volume and tidal volume. Closing volume was only slightly increased, but residual volume was considerably increased, by the raised pressure. The desaturation is only partially explicable on a ventilatory basis, and there is probably a component concerned with perfusion, and the absence of perfusion adaptation from hypoxic areas does not appear to accord with current hypotheses.