The ocular circulation consists of the choroidal and retinal vasculature, adequate blood flow to which is essential for the maintenance of visual functions. In a previous study we found that a change in retinal and choroidal blood flows induced by manipulating ventilation is associated with a change in the visual acuity (Hayashi et al., 2011a). Concomitant increases in both retinal blood flow and contrast sensitivity have been noted in healthy subjects after the administration of sildenafil (Sponsel et al., 2000).
It has been demonstrated that submaximal exercise increases the choroidal blood flow velocity relative to the increase in mean arterial pressure (MAP) (Hayashi et al., 2011b; Ikemura et al., 2011). However, the increase in choroidal blood flow velocity was suppressed, and retinal blood flow was decreased by the hyperventilation-induced decrease in the arterial partial pressure of carbon dioxide (PaC[O.sub.2]) during exhaustive exercise (Ikemura and Hayashi, 2012b). This was explained by a high sensitivity of both ocular blood vessels to variations in the PaC[O.sub.2] (Delaey and Van de Voorde, 2000; Geiser et al., 2000; Harris et al., 1995; Sponsel et al., 1992).
Heat stress could also directly influence the ocular circulation via physiological changes such as to MAP, sympathetic nerve activity, cardiac output and PaC[O.sub.2] (Cui et al., 2010; Keller et al., 2006; Roddie et al., 1956; Rowell et al., 1969). However, the retinal and choroidal blood flow responses to exhaustive exercise under heat-stress have yet to be determined. Heat stress could decrease the retinal and choroidal blood flow responses to exercise by inducing hyperventilation, and a consequent reduction in PaC[O.sub.2] (Rowell et al., 1969). A combination of exhaustive exercise and heat stress could result in additional reduction to the PaC[O.sub.2]. This raises the possibility that the greater reduction in PaC[O.sub.2] could strong vasoconstriction in both retinal and choroidal vasculatures since both ocular blood vessels are sensitive to variations in PaC[O.sub.2]. Thus, we hypothesized that exhaustive exercise under heat condition induces further decreases in both ocular blood flows.
A pressor response, i.e., increase in blood pressure attenuated by heat stress can be another factor decreasing retinal and choroidal blood flows further during exhaustive exercise under the heat condition. Previous studies have reported that heat stress attenuates the pressor response induced by an exercise (Nybo et al., 2002). The pressor response could be attenuated by heat stress during exhaustive exercise. Changes in the MAP affected the choroidal circulation (Hayashi et al., 2011b; Ikemura et al., 2011), and although the retinal circulation is affected less by MAP fluctuations due to the existence of autoregulation, both ocular blood flows may be influenced by attenuation of the pressor response during exhaustive exercise in the presence of heat stress. Ikemura et al. (2012b) demonstrated that hypocapnia decreases the blood flow in both ocular blood flows during exhaustive exercise, in spite of retinal autoregulation. It is therefore possible that attenuation of the pressor response by heat stress induces a further reduction in retinal and choroidal blood flows during exhaustive exercise under the heat condition.
The purpose of the present study was to test the hypothesis that the greater reduction in PaC[O.sub.2] and/or attenuation of the pressor response, which induced by heat stress decrease ocular blood flow during exhaustive exercise. Toward this end, the ocular blood flow response to an exhaustive exercise was assessed under both control and heat conditions.
Twelve healthy males [age, 25 [+ or -] 1 years (mean [+ or -] SD); height, 1.72 [+ or -] 0.02 m; body mass, 67 [+ or -] 3 kg] participated in this study. All of the subjects were free of any known autonomic dysfunction and cardiovascular and ocular disease, and were not taking any medications. The Ethics Committee of the Institution of Health Science, Kyushu University, Japan, approved the experimental protocol, and all subjects provided written informed consent to participate prior to the commencement of the study. All of the protocols used conformed to the Declaration of Helsinki. Each subject visited the laboratory before taking part in the experiments for familiarization with the techniques and procedures of the protocol.
The subjects arrived at the laboratory after having abstained from caffeinated beverages and strenuous exercise for 6 h, and from eating for at least 2 h. The individual target work rate at 75% of their maximal heart rate (153 [+ or -] 7 W) was determined, using an incremental cycle ergometer test in the control condition (20[degrees]C) at least 7 days prior to the experiment. On two separate experimental days, the subjects performed the exercise on a cycle ergometer until exhaustion in control or heat (35[degrees]C) conditions. The order of the thermal condition was randomized. After a 3-min resting period in both conditions, the subjects began cycling at a half of the target work rate. At 1 min after the exercise onset, the exercise intensity was increased to the target work rate. The exercise was continued until the subjects could no longer maintain a pedaling cadence of 60 rpm, or could no longer fix their body trunk to allow acquisition of the ocular blood flow data. During ocular blood flow measurement, the subjects were permitted to some bulr their body trunk, since this did not affect the blood flow analysis. The analyzer software is able to identify the blood vessels to estimate the ocular blood flow at the same target areas each time by identifying bifurcations of retinal arteries as markers. This exercise was followed by a resting recovery period.
The blood pressure and heart rate (HR) were recorded continuously throughout the trial. The ocular blood flow velocity, external ear temperature and respiratory variables were obtained every 3 min during the resting, exercise, and recovery periods. Subjects were asked to open their right eye without blinking for 4 s during the image recording for ocular blood flow measurement. Three laser-speckle images were obtained for the right eye. Subjects were asked to keep their face motionless in front of the apparatus for laser-speckle flowgraphy (LSFG) apparatus while laser-speckle images were obtained. Subjects who normally wore glasses or contact lenses removed them before the experiment. The subjects did not receive any drugs, such as for mydriasis.
The beat-by-beat blood pressure was monitored with an automatic sphygmomanometer attached to the left middle finger (Finometer, Finapres Medical Systems, Amsterdam, The Netherlands). The ECG was recorded continuously using a bioelectrical amplifier (MEG2100, Nihon-Kohden, Tokyo, Japan)....