Supervisors info:
Κοσκολού Μαρία, Αναπληρώτρια Καθηγήτρια, ΣΕΦΑΑ, ΕΚΠΑ
Γελαδάς Νικόλαος, Καθηγητής, ΣΕΦΑΑ, ΕΚΠΑ
Κλεισούρας Βασίλης, Ομότιμος Καθηγητής, ΣΕΦΑΑ, ΕΚΠΑ
Summary:
The immense variation that exists within the population regarding many morphological and functional (physiological) traits could be partly explained by the variance due to genotypic differences and partly explained by the variance due to different environmental exposures. The twin model has been widely used in the biological sciences as a means of estimating the percentage contribution of the genotype to the manifestation of a phenotype.
Prolonged bouts of voluntary breath-holding (apnea) take place in various sporting, recreational and occupational activities. The physiological responses that occur during apnea have been vastly examined and show great diversity in many studies. Maximal breath-hold duration depends on psychobiological factors and the magnitude of the underlying physiological responses. The basic physiological adjustments that occur during apnea, collectively called the mammalian diving response, include vagally mediated bradycardia and sympathetically mediated peripheral vasoconstriction. Although a few genes have been found to be associated with some of the cardiovascular responses during apnea, the extent to which genetic predisposition determines individual differences regarding the diving response has not been studied.
Thus, the purpose of the present study was to estimate the heritability index (h2) of the individual differences in maximal apnea duration and its associated physiological responses. Forty male twins (10 monozygotic and 10 dizygotic twin pairs) aged 24.6 ± 4.8 years were recruited. At first, a detailed questionnaire was administered regarding their levels of daily physical activity and health history, followed by body and vital capacity measurements. They then performed five repeated apneas with the face immersed in cold water. Apnea duration and the cardiovascular parameters: i) heart rate (HR), ii) stroke volume (SV), iii) cardiac output (CO), iv) systolic pressure (SP), v) diastolic pressure (DP), vi) mean arterial pressure (MAP) vii) total peripheral resistance (TPR) were measured throughout the experimental protocol. In addition, oxygen saturation (%SpO2) during apneas, and the recovery thereof, as well as blood lactate levels before and after apneas were recorded.
Regardless of twin type, mean apnea duration increased by 63.4% (p < 0.001) in the fifth compared to the first apnea bout. Heart rate during apnea decreased by 28.3% (p<0.001) compared to resting values. Systolic, diastolic and mean arterial pressure in the final stages of maximal apnea increased (p<0.001) by 40.6%, 50.2% and 47.2%, respectively, compared to resting values. Lactate concentration 3 minutes after the end of the fifth apneic bout increased by 0.3 ± 0.4 mmol/l (p<0.05) compared to rest. By applying the twin model, the heritability indices of individual differences in maximum apnea duration (h2=0.82), mean arterial pressure response (h2=0.73) and vital capacity (h2=0.75) were estimated. However, statistical tests of the hypotheses for derivation of the genetic variance did not allow estimation of the heritability index for the cardiovascular parameters HR, SP, DP, SV, CO and TPR.
In conclusion, it appears that a significant portion of the variability that exists in maximal apnea duration, vital capacity, and mean arterial pressure response during apnea can be explained by genetic differences. The non-significant genetic variance found for CO, HR, SV and TPR could be attributed to the redundant nature of the mechanisms that control MAP, whereby the similar MAP responses could be achieved, at least during apnea, via divergent responses of CO (HR and SV) and TPR.
