Cutaneous Blood Flow and Temperature	Adrenoceptors	Hypothalamic Thermal Stimulation
Used with permission from  Allsorts' Dog Pictures

Effects of Cutaneous Blood Flow on Temperature

• Dogs and other homeothermic animals use regulation of cutaneous blood flow as a method of thermoregulation.  The most effective sites for thermoregulation by regulation of cutaneous blood flow are those areas less insulated.  In dogs these areas include the ears, abdominal and genital regions, which are less covered by fur.  Cutaneous blood flow can be regulated by vasodilation or vasoconstriction of blood vessels near the skin's surface.  Vasodilation of cutaneous blood vessels increases cutaneous blood flow while vasoconstriction decreases cutaneous blood flow.  Increased cutaneous blood flow causes a loss of heat because more blood is flowing to the surface of the body where it loses heat more rapidly to the environment than blood near the body core.  Vasoconstriction has the opposite effect due to decreased blood flow to the surface of the body (Peter and Riedel 1982).

Adrenoceptors and Cutaneous Blood Flow

• Adrenaline and noradrenaline are hormones secreted by the adrenal medulla.  Adrenaline affects what is known as the fight-or-flight syndrome.  Its effects include increased heart rate and blood pressure, increased blood sugar, increased blood flow in heart muscle, lungs and skeletal muscle, and vasoconstriction and decreased blood flow in smooth muscle, skin, and the digestive tract.  Noradrenaline has many of the same effects as adrenaline, although it has a lesser effect on heart rate and a greater effect on vasoconstriction in some organs than adrenaline.  Adrenaline and noradrenaline act through receptor sites known as adrenoceptors (Schmidt-Nielson 1997).
• Adrenoceptors are membrane bound receptors located throughout the body.  They mediate many responses to adrenaline and noradrenaline.  Adrenoceptors can be divided into two groups, alpha and beta, that can be further divided into subtypes.  Alpha-adrenoceptors are found pre and post synaptically in the central and peripheral nervous system, and on vascular and non-vascular smooth muscle, the heart, and the liver.  In the central nervous system alpha-adrenoceptors play a role in regulation of neurotransmitter release, blood pressure, hypothermia, pupil diameter, and central cardiovascular responses.  In the peripheral nervous system their functions include contraction of vascular smooth muscle, inhibition of lypolysis, and hyperpolarization of sympathetic ganglia.  They also increase muscle contraction in the heart and stimulate glycogen phosphorylation in the liver.  Beta-adrenoceptors are found in the central nervous system, in the heart and on vascular, uterine, and airway smooth muscle.  Activation of beta-adrenoceptors results in increases in heart rate and force of contraction of cardiac muscle, vasodilation, and bronchodilation.  Adrenoceptors affect thermoregulation primarily through their effects on changes in cutaneous blood flow.  Generally alpha-adrenoceptors cause decreased cutaneous blood flow through vasoconstriction and beta-adrenoceptors cause increased cutaneous blood flow through vasodilation (Robinson and Hudson 1998).  Dogs infused with beta-adrenoceptor agonists show increased heart rate and cutaneous blood flow and decreased arterial blood pressure.  These effects are due to vasodilation caused by stimulation of beta-adrenoceptors (Berlan et al. 1994).

• Cutaneous blood flow can also be regulated by thermal stimulation of the hypothalamus.  This regulation occurs without adrenaline or noradrenaline.  In dogs heating of the hypothalamus can cause vasodilation which leads to an increase in cutaneous blood flow and a decrease in cutaneous vascular resistance.  Hypothalamic cooling can cause a reduction in cutaneous blood flow  caused by vasodilation (Peter and Riedel 1982).

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