Regulation of peripheral circulation: introduction Ion channels, membrane potential & vascular tone. Intrinsic control of resistance vessels Metabolic control Autoregulation Endothelial factors Extrinsic control of resistance vessels RMP = resting membrane potential VSM = vascular smooth muscle

Peripheral resistance vessels are regulated by intrinsic (local) mechanisms, and by extrinsic mechanisms (hormones & the autonomic NS). Distribution of blood flow among different organs is regulated by myogenic, metabolic, neural, & hormonal effects on arteriolar radius. Extrinsic actions are superimposed on intrinsic control. F =  P/R CO = MAP/TPR MAP = CO x TPR Voltage gated Ca ++ channels in VSM HR x SV Endothelial factors metabolism Autoregulation (myogenic, metabolic) Intrinsic Arteriolar tone Cardiovascular reflexes Hormones Central nervous system MAP = CO x TPR Extrinsic

Excitation contraction coupling in vascular smooth muscle SERCA = sarcoplasmic reticulum Ca ++ ATPase Ca ++ Extracellular Ca ++ Ca ++ stores Ca ++ Ryanodine receptor (SR Ca ++ release channel) Contractile mechanism Sarcoplasmic recticulum SR Ca ++ ATPase Ca ++ Na + L-type Ca ++ channels (dihydropyridine receptors) are voltage gated, open with depolarization of cell membrane No T tubules and no fast Na + channels. Ca ++ enters cells via L-type Ca ++ channels.

Membrane potential of VSM controls cell [Ca ++ ] via voltage gated Ca ++ channels Vascular smooth muscle (VSM) exhibits Intrinsic tone independent of nervous or hormonal input Sustained graded contraction without action potentials. Level of intrinsic tone is directly related to resting membrane potential (depolarization increases tone). Nervous & hormonal control is superimposed on intrinsic tone Open probability of Ca ++ channels Resting Membrane Potential (RMP) L type voltage gated Ca ++ channels are activated by depolarization. VSM RMP = - 40 to - 55 Mv RMP is a mostly a K + diffusion potential.

Contraction of VSM depends on intracellular [Ca ++ ] open K + channels  K + efflux hyperpolarization inactivates voltage gated L type Ca ++ channels  cell [Ca ++ ] vasodilation close K + channels  K + efflux depolarization activates voltage gated L type Ca ++ channels  cell [Ca ++ ] vasoconstriction E K+ = - 84 mV E Ca++ = mV RMP = – 40 to – 55 mV Level of contraction of VSM is set by intracellular [Ca ++ ] Ca ++ enters VSM cells through voltage gated L type Ca ++ channels K+K Ca ++ Vascular smooth muscle cell Ca ++ K+K+

Vasoactive hormones & voltage gated L type Ca ++ channels Vasoconstrictors either Open Ca ++ channels directly or Depolarize the cell membrane which opens Ca ++ channels. Vasodilators either close Ca ++ channels directly or hyperpolarize the cell membrane which closes Ca ++ channels.

K + channels in vascular smooth muscle (VSM) ATP sensitive channel (K ATP channel) ↓ [ATP] or ↑ [ADP] → ↑ open probability Links metabolism to blood flow Responds to vasoconstrictors and vasodilators Contributes to resting membrane potential (some K ATP channels are open under resting conditions) Voltage gated K + channel (Kv) Depolarization → ↑ open probability May contribute to resting membrane potential Inward rectifying K + channel (Kir) At ECF [K + ] above normal, Kir channels open Contribute to vasodilation in muscle during exercise Despite their name, Kir channels allow outward diffusion of K + under physiological conditions.

K ATP channel  metabolism hypoxia K+K+  ATP,  ADP activates Activity of the K ATP channels links metabolism to blood flow Hypoxia or increased metabolic rate activate the K ATP channel.  K + efflux hyperpolarizes the cell membrane. Voltage gated Ca ++ channels are inactivated. VSM dilates. The SUR domain on the K ATP channel is a sulfonylurea receptor or ABC cassette (adenosine binding cassette). K ATP channel can bind ATP or ADP ATP K ATP channel ADP inactivatesactivates

Definition of metabolic control Local blood flow is regulated by the local metabolic level of the tissue. Increased metabolism produces vasodilators that cause an increase in flow. The increased flow increases delivery of O 2 & nutrients and the removal of CO 2 & waste products to match the new level of metabolism. Possible vasoactive metabolites include: carbon dioxide, H +, nitric oxide, adenosine, inorganic phosphate ions, interstitial osmolality.

Local metabolic control of blood flow  Metabolism has the opposite effect Metabolism drives blood flow. Adenosine may be an important regulator of coronary flow  metabolism  tissue ADP, CO 2, H +, lactate, adenosine,  O 2 vasodilation  blood flow open K ATP channels Hyperpolarize cell membrane Close voltage gated Ca ++ channels

Kir channels contribute to vasodilation during exercise During heavy exercise ECF [K + ] increases as K + diffuses out of cells during repolarization. Kir channels: Despite their name, these channels allow outward diffusion of K + under physiological conditions.  skeletal muscle contraction  ECF [K + ] vasodilation  blood flow open Kir channels Hyperpolarize VSM cell membrane Close voltage gated Ca ++ channels

K ATP channels are also affected by hormonal signals K ATP channel Vascular smooth muscle cell  metabolism hypoxia vasodilators receptor K+K+  ATP,  ADP cAMP Gs vasoconstrictors receptor PKC - + Nitric oxide cGMP ++

Hormones that act via K ATP channels Vasodilators Adenosine (coronary & renal circulation) Epinephrine Nitric oxide ANP (atrial natriuretic peptide) Vasoconstrictors Angiotensin II Vasopressin Endothelin

Pressure-flow relationships Flow Pressure Autoregulation dilation constriction Autoregulation: constancy of blood flow when arterial pressure changes. Flow Pressure Passive system Large veins exhibit a passive pressure flow relationship. Flow Pressure Rigid system Doesn’t occur in the circulation.

Myogenic mechanism of autoregulation An example of autoregulation in vivo: Standing up increases arterial pressure in the legs as a function of distance below the heart. Myogenic autoregulation constricts arterioles below the heart. Constriction maintains flow relatively constant Also, this myogenic response prevents an increase in capillary pressure & prevents pedal edema. Myogenic autoregulation is especially effective in the kidney.  pressure Stretches arterioles constriction constant flow voltage gated Ca ++ channels open cell depolarizes Activates nonspecific cation channel in VSM cell Na + enters cell Autoregulation: constancy of blood flow when arterial pressure changes.  Pressure has the opposite effect

Metabolic mechanism of autoregulation Decreased blood pressure and flow have opposite effects.  Blood pressure  Blood flow  Tissue PO 2  Tissue metabolic vasodilators Vasoconstriction  Metabolic vasodilators include: low O 2, high CO 2, [H + ], adenosine, PO 4, interstitial osmolality. Autoregulation is a response to changes in blood pressure. Metabolic control is a response to changes in tissue metabolism.

Flow, ml/min Pressure, mm Hg occlusion Reactive hyperemia Occlusion of an artery is followed by an increase in blood flow above control level when the occlusion is released. The longer the occlusion, the greater the increase in flow. During occlusion hypoxia prevails and vasodilator metabolites accumulate. When flow resumes these metabolites produce vasodilation.

Catecholamines regulating the circulation: norepinephrine Skeletal muscle and coronary arterioles have beta receptors. They can be dilated by low dose epinephrine. Norepinephrine is a constrictor because it has a greater affinity for alpha receptors but it can react with beta receptors. NeuroeffectorReceptor Action NorepinephrineAlphaVasoconstriction NorepinephrineBeta 1  Heart rate  Contractility  Cardiac output

Catecholamines regulating the circulation: epinephrine Secreted from the adrenal medulla. Release controlled by sympathetic nerves Cardiac actions  Heart rate  Contractility  Cardiac output Vascular Constricts Kidney Splanchnic bed Skin Dilates heart & skeletal muscle (low dose)

Angiotensin & vasopressin Angiotensin II Generated by angiotensin converting enzyme in blood in response to secretion of renin from kidneys. Renin secretion is stimulated by:  Arterial pressure or blood volume Low salt diet Vasoconstrictor, increases TPR Retains salt (kidneys) Vasopressin Secreted from posterior pituitary in response to  Arterial pressure or blood volume Dehydration Pain Fear Vasoconstrictor Retains water (kidneys) Vasoconstricting effects of sympathetic nerves, angiotensin and vasopressin are synergistic

Parasympathetic effects on blood flow NeuroeffectorReceptorAction AcetylcholineMuscarinic  Blood flow Salivary glands Gastrointestinal glands Erectile tissue Effects of acetylcholine on blood flow are indirect. Ach acts on the endothelium to release nitric oxide. NO diffuses to VSM & is a vasodilator.

Endothelium, shear stress & nitric oxide synthesis Vascular smooth muscle relaxation  Flow   Shear stress Ca ++ L-arginine Ca ++ + Calmodulin Nitric oxide synthase Nitric oxide (NO) Blood Flow NOS is a calmodulin - dependent enzyme endothelium Nitric oxide is needed for maintenance of normal blood pressure. Pharmacological inhibition of nitric oxide synthase increases MAP into the hypertensive range.

Circulatory response to hypotension  Sympathetic nerve activity Arteriolar constriction  total peripheral resistance  flow (GI tract, kidney, liver, resting muscle). Baroreflex  secretion of epinephrine Constriction: skin, GI tract, kidney Dilation: skeletal muscle, heart Hypotension, hemorrhage, dehydration, pain, fear + Maintain systemic arterial pressure. +

Integration of sympathetic and metabolic control of circulation In general: At rest, blood flow is controlled primarily by a low level of sympathetic tone. Increased work (muscle contraction, secretion, digestion, absorption etc) increases tissue metabolism. Blood flow increases to match the new level of metabolism. Blood flow decreases in inactive tissue due to increased sympathetic tone. Effects of epinephrine on vascular smooth muscle are less important than sympathetic activity.

Aim: to measure upper leg muscle work and metabolism during leg exercise. Cycle ergometer sets work intensity Blood pressure cuff minimizes flow to lower leg Femoral arterial & venous blood sampled

Upper Leg QO 2 versus work intensity Work Intensity, Watts Upper Leg QO 2, L/min Work drives oxygen consumption

Graph shows leg blood flow as a function of work intensity Maximal cardiac output is the limiting factor in aerobic exercise. Upper Leg Blood Flow, L/min Work Intensity, Watts VO 2 = (F)(O 2A - O 2V ) VO 2 may be increased by increasing flow & increasing oxygen extraction Maximum flow = 2L/min per kilogram of muscle. If projected to the whole body, this flow would be equivalent to CO = L/min. The capacity of skeletal muscle to receive blood is greater than the maximal cardiac output.

VO 2 is increased by increasing oxygen extraction and blood flow Femoral A -V O 2 Difference versus work intensity Work Intensity Femoral A-V difference, mlO2/100 ml blood Exercising the other leg: decreases flow increases extraction When both legs are exercised, flow is controlled by the balance of opposing sympathetic and metabolic effects VO 2 = (F)(O 2A - O 2V )O 2A - O 2V

Summary Arteriolar resistance determines distribution of flow between organs. Vascular smooth muscle (VSM) has basal tone independent of nerves & hormones. Tone of VSM is regulated by gradual changes in RMP & cell [Ca ++ ]. Stretch depolarizes VSM & increases tone (myogenic response). Increased local metabolism dilates VSM (metabolic regulation). Autoregulation maintains constant flow when pressure changes (brain, heart, kidney, skeletal muscle). Local metabolic control predominates in heart & brain. Muscle blood flow in active tissue is a balance between sympathetic (constrictor) & metabolic (dilator) effects. Neural control predominates in the splanchnic region and skin. Sympathetic nerves, angiotensin II and vasopressin potentiate each other’s effects.