Blood flow to the brainstem and a possible link to high blood pressure (essential hypertension) Dr Phil Langton

Resting heart rate Max heart rate Resting CO Max CO Miguel Indurain ~30 bpm 200 bpm 5 litres 50 litres

Blood flow Two circuits Requires work (not passive) Distribution is dynamic Distribution has multiple functions Distribution is regulated How is blood flow regulated? – Global vs regional (or local) flow…….

Resistance to flow MAP changes are small Cardiac output changes ~5-fold BP=CO.TPR So, TPR must alter in proportion to CO Qu. What are the units of TPR????

Vessel calibre and resistance Resistance - Proportional to length Proportional to calibre – Resistance related to r 4 Is the calibre of an artery supplying resting muscle… a)Fully relaxed? b)Fully contracted?

Rates of flow: resting to fully activated Consider: The sum of maximal flow rates exceeds max cardiac output! Discuss the implications of this…

Determinants of Blood Pressure MAP = CO x R

The ‘myogenic contraction’ of small arteries – Described by Bayliss (1902) – Thought important for autoregulation Defined as ‘the tendency for local tissue blood flow to be independent of systemic blood pressure’ - s mall arterial blood vessels - autoregulatory range (in man) mean blood pressure mean blood pressure (mmHg) blood flow (ml/min/100g) Dilated: low resistance Constricted: high resistance Arterial cross section

Myogenic contraction [of small arteries] An isolated cannulated artery (~0.2 mm dia.) l Typical myogenic contraction in absence of calcium or at room temperature in presence of calcium (normal myogenic response) Diameter (normalised) internal pressure (mm Hg) threshold pressure 190  m 295  m

Myogenic depolarisation From Knot and Nelson, (1998), J. Physiol. 508: Characteristic feature of myogenic constriction

Ca source in muscle SR = Intracellular calcium store Skeletal muscle smooth muscle

Ca source in muscle Ca SR = Intracellular calcium store Skeletal muscle smooth muscle Ca entering through Ca channels

Likely importance of depolarisation Steady-state Po Membrane potential (mV) From Nelson et al., Am. J. Physiol Voltage-dependence of steady state open probability of calcium channels

Likely importance of depolarisation Membrane potential (mV) Steady-state Po Voltage-dependence of steady state open probability of calcium channels [expanded voltage scale] From Nelson et al., Am. J. Physiol. 1990

A few important facts Myogenic constriction of SMALL arteries – Not seen in arteries over a given size Assumption: myogenic mechanism is present or absent Threshold pressure for myogenic constriction – Little evidence below 40 mmHg Temperature-sensitive – Myogenic constriction - absent or very attenuated at room temp – Temperature dependence of myogenic depolarisation has never been examined…….

Passive Diameter at 80 mmHg (m) Normalised arterial diameter at 80 mmHg (% of passive diameter)  Rat mesenteric Smaller arteries have more pronounced myogenic response

Passive Diameter at 80 mmHg (m) Normalised arterial diameter at 80 mmHg (% of passive diameter)  325 micrometers No myogenic tone Rat mesenteric

But muscle rmp still varies with diameter Passive Diameter at 80 mmHg (  m) Resting membrane potential at 20 mmHg (mV) Not myogenicmyogenic Non-myogenic arteries (>320µm) 20mmHg80mmHg M e m b r a n e p o t e n t i a l ( m V ) And even larger arteries depolarise to pressure 20 mmHg used as below threshold for myogenic response Rat mesenteric

Temperature – myogenic depol absent at 22 o C 22 ° C 37 ° C Pressure (mmHg) M e m b r a n e P o t e n t i a l ( m V ) Pressure (mmHg) M e m b r a n e P o t e n t i a l ( m V ) B (I) (II) ***** CerebralMesenteric A Pressure Temperature Membrane Potential 20 mmHg 80 mmHg 20 mmHg 80 mmHg -46 mV-45 mV-44 mV-30 mV-32 mV 10 mV 100 s 22 o C37 o C22 o C37 o C 0 mV

Interpretation & conclusions Myogenic constriction and depolarisation are lost at room temperature Larger arteries are not ‘myogenic’ but do depolarise when pressured The more negative resting potential of larger arteries may explain their lack of response to pressure.

End of part 1 Questions? ‘Blood flow to the brainstem and a possible link to high blood pressure (essential hypertension)’, Coming next part 2

Average resting blood pressure is 120/80 (mmHg) – Hypertension = systolic above 140 or diastolic above 90 mmHg – ~1/3 people in England have hypertension % with high blood pressure Men Women 31% 28% Data source: Half of those treated remain hypertensive [Essential] Hypertension (EH)

Sympathetic nervous system in EH Grassi G

Why Skeletal Muscle Flow? ~40% of body mass is skeletal muscle (skm) Resistance to flow altered to manage changes in MAP (e.g. during posture changes) Large variation in flow ~100 fold change Known association between exercise, oxygen use and muscle BF Requirement for targeted blood flow – to active (& not inactive) muscle

There is a Graded Association Between Hypertension and Sympathetic Drive (Grassi 1998) Normotensive Mildly Essential Hypertensive Severe Essential Hypertensive Secondary Hypertensive Mean Arterial Pressure (mmHg) (Grassi J Hypertens, 16: ) Symp Nerve Activity (bursts per 100 heart beats)

Harvey Cushing (Baltimore, 1903) The Cushing Mechanism & Neurogenic Hypertension conscious dog systemic arterial pressure intra-cranial pressure The Cushing Response (1901) MAP mmHg Intra-Cranial Pressure mmHg (Bull.Johns Hopk. Hosp., 12: ). cerebral vascular resistance

Vertebral Artery Flow & Mean Arterial Pressure Mean Ante- Mortem Blood Pressure (mmHg) Rate of Flow in Both Vertebral Arteries (ml per second) Dickinson (1960) J. Clin. Sci.,19, P If radius reduced by half, resistance increases by 16-fold Q= P/R R α 1 r 4

vertebral arteries Dickinson (1965). Neurogenic Hypertension. Blackwell Normotensive Hypertensives Smaller Diameter Vertebral Arteries in Humans With Hypertension

Animal Model of Human Hypertension: The Spontaneously Hypertensive Rat (SHR) Genetically pre-programmed hypertension Dependence on renin-angiotensin system Responsive to human anti-hypertensive medication Does it have narrowed cerebral vessels?

SHR & WKY Rats WKY = Normotensive Control Rats SHR = Spontaneously Hypertensive Rats = SHR ⁰ = WKY (Dickhout & Lee 1998 Am. J Phys 43: )

Hypothesis There is a difference in cerebrovascular architecture consistent with high vascular resistance in pre-hypertensive SHRs.

Method Method used: Kruker et al., Microscopy research and technique 69:138–147 Pentabarbitone overdose (IP) Cannulation of left ventricle Perfusion (20-24 o C): Saline flush + hydralazine Fix – 100ml 4% formalin Resin (Pu4ii, Vasqtec) Maceration (KOH / acetic acid) Freeze dried; sputter coated Visualised in SEM (5kV) Detergent washes and rinsing 1 hour 10 days Curing at 10 o C24 hours

Left vertebral Right vertebral Basilar caudal rostral

Basilar

Basilar artery of SHR smaller 63% reduction in conductance

Vascular Maps of SHR and WKY WKYSHR

BASILARBASILAR re. to WKY LEFT VERTEBRAL rel. to WKY RIGHT VERTEBRAL rel. to WKY CONDUCTANCE  RADIUS 4  Conductance of feeding vessel taken as 1  Conductance of SHR appears lower than WKY Decr Cond.

Summary & Conclusions  SHR has smaller median basilar diameter  SHR basilar - more heavily branched  Right vertebral of SHR distinctly small  Right vertebral of SHR more heavily branched than left in SHR and WKY  Estimated conductance of SHR lower than WKY, especially if normalised to WKY  Differences are apparent between vascular casts of SHR and WKY that are consistent with higher cerebrovascular resistance in the SHR

Questions?