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J. C. B. Jacobsen and C. Aalkjær
(although with different amplitude relative to baseline). Others have identified oscillations in the rat testis with similar characteristics, and, notably, such oscillations are
local and vary in characteristics across the tissue [8]. As pointed out by Lysiak et al.
[37], the normal interstitial oxygen pressure of the testis is very low (in the range of
half of that found in other tissues); probably the testis interstitium is a protected environment, also when it comes to avoiding high oxygen levels which might increase
production of reactive oxygen species.
These observations raises the question if, on a theoretical basis, the presence
of flowmotion per se is beneficial to the tissue. Goldman and Popel [15] simulated the effect on tissue oxygenation of oscillations in blood flow velocity, in a
network of skeletal muscle capillaries. Under “resting” conditions when the amount
of inflowing oxygen is plenty relative to oxygen consumption rate, there is no effect
of oscillations in flow velocity (in any combination of amplitude and frequency) and
throughout the tissue oxygen tension remains sufficient. Increasing tissue metabolic
rate while preserving the same flow increases heterogeneity in tissue oxygenation
since the delivery of oxygen is not uniformly efficient in a network with a naturally
heterogeneous structure. In this situation, flowmotion appear to alleviate the hypoxia
of undersupplied regions. The most efficient frequency in this system was in the
range 1.5–3 cpm in agreement with the abovementioned observations from skeletal
muscle [50] and skin [52]. Introducing an oxygen buffer into the model in the form
of myoglobin removes the beneficial effect of vasomotion, however as pointed out
by the authors, many tissues have little or no myoglobin.
There are indications that vasomotion may also promote drainage of the interstitium. Sakurai and Terui [48] induced vasomotion in the rabbit ear microcirculation
by electrical stimulation of the cervical sympathetic nerve and studied the clearance
of radioactive Cr-EDTA injected into the interstitium. Stimulating the nerve while
keeping the ambient temperature in the interval 25–35 °C consistently induced vasomotion (around 3 cpm) and a reduction in total flow. Below or above this temperature
interval, stimulation caused the same reduction in total flow but without vasomotion.
The radioactive isotope was however, cleared from the tissue at a double rate in
the presence of vasomotion. Also based on calculations of transcapillary fluxes it
has been suggested that during pressure fluctuations caused by vasomotion there
will be an increased drainage of the interstitium [27]. As pointed out by Intaglietta [28] an increased drainage is required to increase intravascular volume during
systemic hypotension which, as outlined above, is a state associated with increased
prevalence of vasomotion. Likely, vasomotion is further stimulated in this situation
due to release of the vasoactive substance vasopressin [14]. There is, however, a
general need for experiments, which can critically test the hypothesis that vasomotion acts to preserve tissue homeostasis and functionality by enhancing uniform
oxygen delivery and removal of waste products. Ideally, this would be experiments
that could report tissue parameters that reflects deviation from normal tissue homeostasis under various conditions. The parameters measured could be e.g. oxygen- and
carbon dioxide pressures, pH, interstitial lactic acid and potassium concentration etc.
but also tissue function such as contractility or conductivity in electrically coupled
tissues. The conditions investigated could be a gradual reduction in tissue perfusion
J. C. B. Jacobsen and C. Aalkjær
(although with different amplitude relative to baseline). Others have identified oscillations in the rat testis with similar characteristics, and, notably, such oscillations are
local and vary in characteristics across the tissue [8]. As pointed out by Lysiak et al.
[37], the normal interstitial oxygen pressure of the testis is very low (in the range of
half of that found in other tissues); probably the testis interstitium is a protected environment, also when it comes to avoiding high oxygen levels which might increase
production of reactive oxygen species.
These observations raises the question if, on a theoretical basis, the presence
of flowmotion per se is beneficial to the tissue. Goldman and Popel [15] simulated the effect on tissue oxygenation of oscillations in blood flow velocity, in a
network of skeletal muscle capillaries. Under “resting” conditions when the amount
of inflowing oxygen is plenty relative to oxygen consumption rate, there is no effect
of oscillations in flow velocity (in any combination of amplitude and frequency) and
throughout the tissue oxygen tension remains sufficient. Increasing tissue metabolic
rate while preserving the same flow increases heterogeneity in tissue oxygenation
since the delivery of oxygen is not uniformly efficient in a network with a naturally
heterogeneous structure. In this situation, flowmotion appear to alleviate the hypoxia
of undersupplied regions. The most efficient frequency in this system was in the
range 1.5–3 cpm in agreement with the abovementioned observations from skeletal
muscle [50] and skin [52]. Introducing an oxygen buffer into the model in the form
of myoglobin removes the beneficial effect of vasomotion, however as pointed out
by the authors, many tissues have little or no myoglobin.
There are indications that vasomotion may also promote drainage of the interstitium. Sakurai and Terui [48] induced vasomotion in the rabbit ear microcirculation
by electrical stimulation of the cervical sympathetic nerve and studied the clearance
of radioactive Cr-EDTA injected into the interstitium. Stimulating the nerve while
keeping the ambient temperature in the interval 25–35 °C consistently induced vasomotion (around 3 cpm) and a reduction in total flow. Below or above this temperature
interval, stimulation caused the same reduction in total flow but without vasomotion.
The radioactive isotope was however, cleared from the tissue at a double rate in
the presence of vasomotion. Also based on calculations of transcapillary fluxes it
has been suggested that during pressure fluctuations caused by vasomotion there
will be an increased drainage of the interstitium [27]. As pointed out by Intaglietta [28] an increased drainage is required to increase intravascular volume during
systemic hypotension which, as outlined above, is a state associated with increased
prevalence of vasomotion. Likely, vasomotion is further stimulated in this situation
due to release of the vasoactive substance vasopressin [14]. There is, however, a
general need for experiments, which can critically test the hypothesis that vasomotion acts to preserve tissue homeostasis and functionality by enhancing uniform
oxygen delivery and removal of waste products. Ideally, this would be experiments
that could report tissue parameters that reflects deviation from normal tissue homeostasis under various conditions. The parameters measured could be e.g. oxygen- and
carbon dioxide pressures, pH, interstitial lactic acid and potassium concentration etc.
but also tissue function such as contractility or conductivity in electrically coupled
tissues. The conditions investigated could be a gradual reduction in tissue perfusion