Reflections on the OECD guidelines for in vitro skin absorption studies.

At the 8th conference of Occupational and Environmental Exposure of the Skin to Chemicals (OEESC) (16-18 September 2019) in Dublin, Ireland, several researchers performing skin permeation assays convened to discuss in vitro skin permeability experiments. We, along with other colleagues, all of us hands-on skin permeation researchers, present here the results from our discussions on the available OECD guidelines. The discussions were especially focused on three OECD skin absorption documents, including a recent revision of one: i) OECD Guidance Document 28 (GD28) for the conduct of skin absorption studies (OECD, 2004), ii) Test Guideline 428 (TGD428) for measuring skin absorption of chemical in vitro (OECD, 2004), and iii) OECD Guidance Notes 156 (GN156) on dermal absorption issued in 2011 (OECD, 2011). GN156 (OECD, 2019) is currently under review but not finalized. A mutual concern was that these guidance documents do not comprehensively address methodological issues or the performance of the test, which might be partially due to the years needed to finalize and update OECD documents with new skin research evidence. Here, we summarize the numerous factors that can influence skin permeation and its measurement, and where guidance on several of these are omitted and often not discussed in published articles. We propose several improvements of these guidelines, which would contribute in harmonizing future in vitro skin permeation experiments.

Role of hypoxic pulmonary vasoconstriction in pulmonary gas exchange and blood flow distribution. 2. Pathophysiology.

In this review, the second of a two part series, the analytic techniques introduced in the first part are applied to a broad range of pulmonary pathophysiologic conditions. The contributions of hypoxic pulmonary vasoconstriction to both homeostasis and pathophysiology are quantitated for atelectasis, pneumonia, sepsis, pulmonary embolism, chronic obstructive pulmonary disease and adult respiratory distress syndrome. For each disease state the influence of principle variables, including inspired oxygen concentration, cardiac output and severity of pathology are explored and the actions of selected drugs including inhaled nitric oxide and infused vasodilators are illustrated. It is concluded that hypoxic pulmonary vasoconstriction is often a critical determinant of hypoxemia and/or pulmonary hypertension. Furthermore this analysis demonstrates the value of computer simulation to reveal which of the many variables are most responsible for pathophysiologic results.

Role of hypoxic pulmonary vasoconstriction in pulmonary gas exchange and blood flow distribution. 1. Physiologic concepts.

The regulation of the distribution of ventilation/perfusion ratios by hypoxic pulmonary vasoconstriction contributes to both the efficiency of gas exchange and to pulmonary hemodynamics. In this review, the first of two part series, are summarized the physiologic principles on which the analysis of ventilation/perfusion ratios and of pressure-flow relationships are based. A new combined analysis is introduced that permits the important contributions of hypoxic pulmonary vasoconstriction to overall gas exchange to be demonstrated in the circumstances of clinical complexity.

Chronic stability of bipolar epicardial plunge electrodes in dogs.

We constructed and surgically implanted 114 chronic bipolar epicardial plunge electrodes for programmed left ventricular stimulation in closed chest dogs; 88 electrodes could be analyzed in animals surviving infarct surgery. Electrode plunges were constructed of silver wire, with conduction strands of silver-plated copper wire, in medical grade silicone tubing. Electrodes were implanted epicardially through left thoracotomy and secured with prolene. Wires exited the fifth intercostal space and were tunneled subcutaneously and secured at the dorsal aspect of the neck. Baseline thresholds (mA) were recorded at a paced cycle length of 300 ms and pulse duration of 1 ms. At least 5 days after implantation, under light Nembutal sedation, thresholds were reassessed before programmed stimulation. Each lead was tested repeatedly over 5-177 (mean 28) days. The number of leads decreased with time due to animal attrition from ventricular arrhythmias. Mean pacing threshold at implantation was 0.25 mA. Mean and median threshold values reached plateau after 1 week and showed little change thereafter for the duration of the study. More than 50% of the leads maintained thresholds less than 1.5 mA through the entire study. In animals that survived, 86% of the electrodes remained useful for the duration of the protocol. These data support the use of this electrode system as effective and reliable for chronic electrophysiologic studies in dogs.

The effects of ischemia on metabolism and reperfusion arrhythmias.

In attempts to determine the mechanism(s) underlying reflow rhythm disturbances, we have studied the relationship between extent of coronary flow impairment and incidence of reperfusion arrhythmias. In isolated guinea pig hearts perfused with pyruvate (10 mmol/l) and glucose (0.5 mmol/l), coronary flow was reduced to different extents (18, 11, 6, 1, and 0.5%). Following 10 minutes of ischemia, reflow arrhythmias were quantitated with computer-aided statistical determination of rate-independent variations in beat intervals. The results (19 +/- 1, 13 +/- 5, 22 +/- 4, 8 +/- 3 and 6 +/- 1, n = 6, Rhythm Disturbance Units respectively) revealed that rhythm disturbances were more serious after less severe ischemia than after more severe ischemia. To investigate this "paradoxical" observation, we compared the metabolic changes during ischemia and the severity of subsequent reflow arrhythmias. Electrical instability during reperfusion was not related to accumulation of lactate, increase in cyclic AMP or decline in energy status. These were at a maximum in the severely ischemic myocardium. The reduced incidence of arrhythmias following severe (1% and 0.5% flow) as opposed to moderate ischemia, however, may have been associated with a major increase in glycogenolysis (from 1.2 to 7.4 and 7.6 mumol glucose equivalents/min per g dry weight).