Lecture 1 – Cardiac Muscle Properties and Heart Function
∙ Commonality in skeletal and cardiac muscle ???? striations, same structure in terms of sarcomeres, same contractile proteins (thin filaments made of actin, thick filaments made of myosin), the things holding the actin and myosin in place are exactly the same, at the molecular level the switch for turning it on is the same (tropomyosin covering the binding site), calcium is the activator and regulatory proteins are the switch
∙ Differences between skeletal muscle and cardiac muscle/3 Components of what takes up the volume of a skeletal muscle ???? how we get calcium into the cell, you can tell a lot about a skeletal muscle about what you put in there. Ex: want a strong muscle, put in the finite volume a lot of (1) myosin and actin, more contractile proteins – the strength of a muscle is proportional to the cross-sectional area. (2) Sustainability is largely a function of mitochondria. We need to get our energy/ATP aerobically as opposed to anaerobically. As soon as we put mitochondria in there we take away strength because we take away space for myosin and actin filaments. (3) Sarcoplasmic reticulum – a calcium reservoir. Calcium floods out of the SR down its concentration or electrochemical gradient. The rate limiting step for contracting the muscle over and over again is proportional to the density of the sarcoplasmic reticulum.
∙ Cardiac muscle’s three players ???? SR, myofilaments, mitochondria. In the chart of % of cardiomyocyte volume ???? Nuc = nucleus of the cell.
∙ Differences between cardiac myocytes and skeletal myofibrils
o morphology = much larger nuclear volume in heart muscle (the heart is working continuously, doesn’t shut down like skeletal muscle, constant turn-over of proteins means transcription and translation – need more DNA templates to be constantly making these proteins) – proteins shouldn’t be worn out, want them to be fresh Don't forget about the age old question of Why do cells divide?
▪ Cardiac muscle has huge nuclei (1-2) – large volume but less number – smaller volume cells
▪ Extraordinary volume of mitochondria - mitochondria volume of cardiac muscles is very large – lots of on board ATP manufacturing – means lots of blood vessels and capillaries required
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▪ A tissue designed to work sustainably, forever until you die
▪ Relatively low SR volume – means that the repetitive contraction rate is pretty slow - other sources of calcium
▪ Calcium comes from inside the sarcoplasmic reticulum in cardiac muscle ▪ With a skeletal muscle it can contract without extracellular (ECF) Calcium because the entire calcium store is internal in the SR
▪ Cardiomyocytes depend upon ECF calcium as well as SR calcium for not only contraction, but for depolarization
▪ Calcium still comes from the SR but not via direct contact but from calcium induced calcium release (CICR) – why we don’t get any If you want to learn more check out Who was opposed to the new deal?
contraction in a calcium free medium – need external calcium there
▪ External calcium entering the cell interacts with the ryanodine receptor on the SR which opens the calcium channel (like a ligand gated calcium channel)
▪ 2 things going on – calcium channels on the sarcolemma (SL) where we didn’t have that before (not in skeletal muscle) & across vertebrates, it has changed a bit ???? requires calcium channels on the sarcolemma
∙ Calcium source varies by vertebrate taxa – fish have narrow cardiac myocytes, mammal with thicker myocytes – fish calcium comes from 15-20% in the SR, mammal has 80-85% (SR values) – almost inverse of each other
∙ If you are opening up a channel for calcium, how do you know that it wants to go into the cell? The concentration gradient and driving force, which is calculated with the equilibrium voltage, which is calculated using the Nernst Equation. We need to know what the equilibrium potential for calcium is. [Rtzf Log 2.5/.001 for mammals @37 degrees] 30 * log 25,000 = 4.4. 132 mv equilibrium potential means that things want to move until there is 132+ inside. The estimated resting voltage is -80 or -70. Driving potential is about 200 mv. If you open up calcium channels, things come flying in. The number of channels will determine the actual calcium current. We also discuss several other topics like What is a difference between sound waves and light waves?
∙ Thus for proper cardiomyocyte function you need a predictable ECF calcium concentration.
∙ Therefore blood calcium/ECF calcium is tightly controlled (vitamin D and PTH are key players, calcitonin are involved some too)
∙ The source of our calcium is our diet
∙ Vitamin D is made in our skin through UV radiation, it needs two activating steps in the liver and kidney and helps take up calcium from the diet
∙ More PTH, the more vitamin D active, more calcium you can get out of the diet ∙ PTH dissolves bone
∙ There is calcium coming into the cell. There are no calcium channels ∙ With external calcium channels (L-Type), the depolarization contraction cycle is much longer in cardiac muscle.
∙ L-type calcium channels are slower to inactivate than a sodium channel. They stay open for a long time and gradually close. You get a longer absolute and relative refractory period.
∙ Tetanus in heart muscle cell is bad, it means you’re dead. Tetanus in skeletal muscle cells is good, it means you can lift a chair over your head and hold it there for a long period of time.
∙ Depolarization can be driven solely by calcium channels but also by sodium and calcium. It depends on which cells. It varies even within a heart Don't forget about the age old question of Is aggression innate or learned?
∙ How do you get skeletal muscle to contract? It starts in your brain. The muscle is stimulated by neurotransmitter at the neuromuscular junction. With cardiac muscle it contracts by itself and is automatic.
∙ [Difference] Skeletal muscle fibers need to be stimulated in some manner to contract and depolarize. Cardiac myocytes spontaneously depolarize and then contract
∙ SA node cells are the fastest depolarizing cells. They set the pace for the rest of the heart and are called pacemaker cells.
∙ Start off with repolarization (just like neural and muscle cells) – potassium is leaving the cell, making the cell more negative - moving out to in – the funny current (HCN) is a sodium current, which is initiated by a hyperpolarization/repolarization – this opens a low conductance sodium channel – sodium enters the cell, and the voltage starts creeping up towards threshold – this opens a second calcium channel which is also a low conductance calcium channel that opens in the -70 range (called the calcium transient channel = Calcium T). As it starts depolarizing the cell which opens calcium transient channel – lets calcium in but not in great volume, low conductance. Reaches threshold. [The membrane clot model] Don't forget about the age old question of What is meant by absolute configuration?
∙ Some vertebrates don’t have funny channels, but there was still automaticity. ∙ [The calcium clot model] Already hyperpolarized from potassium, but the big thing that is different now is that there are local calcium releases, little pulses of calcium coming out of the cell. Started hyperpolarization and then the SR releases pulses of calcium which fuel the sodium calcium exchanger that lives on the membrane. These calcium pulses fuel sodium calcium exchange which kicks 1 Ca++ out of the cell for 3 Na enter the cell. With only one positive leaving and 3 entering, it is a depolarizing current, a positive current. When they try to get rid of this, or do block outs they get rid of automaticity and they are often lethal mutants
∙ [Morphology difference] Cardiac myocytes are branched compared to the linear, tubular shape of skeletal muscle myocytes. They are electrically connected to each other through gap junctions. With skeletal muscle, if there are adjacent muscle fibers if you wanted the muscle to contract you would typically have a different neuromuscular junction. Each skeletal muscle had its own neuromuscular junction, but in cardiac muscle you can stimulate one cell to depolarize and it will cause the next cell to depolarize because they’re all connected.
∙ Force generation required forms a net and closing a ball of blood and generates pressure equally from all sides, uniformly. With gap junctions you don’t need a separate stimulus for each cardio myocyte because they’re all connected.
∙ Gap junctions are a little bit different in fish vs. mammal. We have intercalated disks (glued together, multi-desmosome structures) and fish are much more dispersed
Lecture 2 – Control of Cardiac Rate
∙ Heart muscle cells are connected electrically through proteins called connexins. In humans there are 21 different gene varieties. They are hexamers and form disulfide bonds. There are several different types (isoforms). They have open and close states and respond to calcium, pH and voltage of the cell. The state of connexins can change depending on the physiology of the cell. Conductance through the channels varies as cell physiological state varies or changes. They are covalently bonded to each other and
are anchored to each other from adjacent cells. It is a fairly weak bond that can be readily broken.
∙ A fish heart would have the fastest, depolarizing cells in the back and you get a wave traveling at 37 cm/s through the fish heart in the direction where you would want to pump the blood. Mammals and birds evolved a complex four chambered heart, and needed to work in high pressure. We had to evolve a conduction system that would first delay the signal, carry it down there faster and then delay the signal, and then carry it down to the tip first and back up so there is contraction from the bottom up and blood will push out of the top. You get a variance in electrical travel.
∙ SA node cells spontaneously depolarize faster than all of the other cells, and they’re the pacemaker. A wave (from the SA node) will spread through the whole heart before other cells have their chance to spontaneously depolarize. Wave spread/depolarization spread in mammal heart is at a variable rate, so it is location dependent. This differential rate of travel/conduction velocity could depend on different connexins, types and number of gap junctions, types of cells and size of cells (Smaller = slower). The AV node cardio myocytes are much smaller and conduct at much lower rates than the bigger cells down further in the purkinje fibers. If you want something to fast it will be larger with more gap junctions.
∙ The heart needs to respond to load. Spontaneous depolarization and spread of electrical potential changes in response to load. [Delta Heart Rate in response to load] o Increase load, increase heart rate – positive chronotropy (time growth) – when you get a perceived load you get a fight or flight response (ex: a tiger jumping out at you) – this is mediated via the sympathetic nervous system (a branch of the ANS). The Sympathetic Nervous System resides in the paravertebral ganglia that run down either side of the vertebral spinal column.
o The heart is innervated by post ganglionic sympathetic fibers. This doesn’t control automaticity of the heart but it controls the rate at which it happens. These post ganglionic sympathetic fibers release norepinephrine onto the surface of heart muscle cells (this is dumping into the extracellular fluid all around the heart).
∙ The fight or flight has a dual punch to it. The immediate effect is like when a person sat up, their heart rate increased drastically. The longer lasting effect is like not feeling well days before an exam. This is mediated by release of a hormone into the blood stream (epinephrine or adrenaline). The adrenaline comes from sympathetic ganglia but it doesn’t form a synapse there. Spinal nerves come out and go all the way to the adrenal gland without making a synapse. The adrenal gland is basically acting like the post ganglionic synapses but they’re not in a ganglion. Instead of going to the heart or an organ, the product is just dumped into the blood. Adrenaline is released into the bloodstream and it will interact with the heart. Because it’s in the blood it will be a longer lasting, more chronic signal
o Longer lasting sympathetic response curves through the release of epinephrine/adrenaline by adrenal or suprarenal chromaffin cells in response to firing of sympathetic neurons. – this is a longer term response but it is
interacting with the same cellular components – B-adrenergic activation is longer term
o Both interact with a B-adrenergic receptor on the surface of the cell (SA node cell)
o Epi and norephi influence the SA node cell. They are peptide hormones so they’re not going into the cell like a steroid hormone, they’re biding to a receptor on the outside. B-adrenergic receptor is on the cell surface and are GS receptors. It is a stimulatory G-protein receptor and activates cyclic adenylate cyclase (AC) and produces cyclic AMP from ATP. It increases cyclic AMP concentration. [It turns on an enzyme that makes cAMP, and there will be more cAMP as a result].
o More depolarization’s per unit time causes your heartrate to beat faster. o Take home ???? increase the concentration of cyclic AMP and it binds to one of the ion channels – activates sooner and increases the conductance – cyclic AMP binds to sites on the IF/HCN channel and causes it to open earlier which means a less negative voltage and have greater conductance for sodium when open ▪ When sodium is let in sooner it will come in faster, and it can bring it back to threshold faster
∙ Increased cAMP also activates a protein called Protein Kinase A (PKA) and then PKA will phosphorylate the calcium L channels – these were the ones responsible for causing the action potential in SA node cells and a long refractory period, L stands for long-lasting – they drive depolarization
o Phosphorylation of calcium L channels causes them to open earlier at a more negative voltage and to increase their conductance of calcium
∙ We can phosphorylate voltage gated potassium channels to take you back faster (ex: like increasing the size of your engine but not increasing the supply of gas to it) o PKA also phosphorylates voltage gated potassium channels, and this will give us a faster recovery or repolarization – we now have a faster heartbeat – increase the rate of recovery
∙ How does PKA and cAMP influence the calcium cycling part of this? It phosphorylates the ryanodine channels and it causes the calcium sparks that drove the calcium clock, to come out earlier and in greater abundance. It doesn’t do anything to the sodium/calcium exchanger.
∙ B-adrenergic modulation of the calcium clock (time – can’t just modulate part of the clock) activated PKA phosphorylates ryanodine channels causing them to release calcium sparks (local calcium releases/pulsatile calcium releases) sooner which means a less negative voltage and then with more calcium released/more gusto
∙ PKA also phosphorylates phospholamban – an inhibitor of the serca ATPase (causes skeletal muscle to recover from the calcium transient) – it is a calcium ATPase and takes calcium from the cytoplasm and pumps it (at the expense of ATP because it’s against the concentration gradient – active transport) from the cytoplasm into the sarcoplasmic reticulum
o Calcium ATPase pumps 1 calcium per ATP, and is expensive and accounts for 20% of the skeletal muscle ATP that you use.
o Calcium ATPase removes calcium from the cytoplasm to the SR at the expense of ATP
o If we increase the rate by reducing/removing phospholamban inhibition there will be more pumping.
o Increased serca activity will do two things - speed recovery from depolarization (just like the potassium channels did) and increase the SR store of calcium so that release from ryanodine channels is more robust (spraying out instead of trickling out)
∙ Breaks of the heart is mediated by the parasympathetic branch of the autonomic nervous system
o Negative cardiac chronotropy (slowing the heart – less depolarization’s per unit time of SA node cells) and this is accomplished by the parasympathetic branch of the ANS
o The heart is innervated by the vagus nerve which is the 10th cranial nerve ▪ The vagus nerve innervates only two spots in the heart – the SA and AV node – slow the rate of spontaneous depolarization and match it up by slowing down the passage through the passage of the AV node
▪ The parasympathetic only deals with speed and doesn’t have anything to do with force
▪ Release choline onto the heart muscle cells/SA node cells
▪ Less depolarization’s per unit time mean slower heart rate
▪ Same neurotransmitter but a different receptor – the specificity of response in a neural cell relies on the receptor and what its coupled to inside the cell, not the neurotransmitter
▪ Acetylcholine in the skeletal muscle cell will cause depolarization,
contraction of a skeletal muscle ???? activates
▪ Acetylcholine in cardiac muscle will bind to another receptor and slow the heartrate ???? deactivates or slows down – negative chronotropy
∙ Acetylcholine interacts with muscarinic acetylcholine receptor (also a G-protein, but it is inhibitory)
∙ Beta/Gamma subunit activates a potassium channel that increases potassium conductance
∙ More cAMP there will be a greater sodium current through the IF channel, if there is less cAMP there will be less sodium current
o Decrease cAMP = decrease adenylate cyclase activity = decreased sodium current through the IF channel (opposite of adrenaline) – slowing the heart, making things go less fast
o The B-subunit activates the K channels and the alpha subunit decreases the adenylate cyclase activity
∙ cAMP also activates PKA and the calcium channels – if there is less cAMP there will be less activation of the calcium channels – two effects
o calcium doesn’t come in as robustly, as fast
o calcium doesn’t come in as far
o decreased calcium current through calcium L channels
∙ inhibit with the vagus nerve and acetylcholine